Solution:

The person on 73rd number will survive.

Explanation:
Till the number is the power of 2, the last person to survive will be the one who started it. But since the number here is not the power of 2, we will take the greatest power of 2 that is less than the number which is 64.
100 - 64 = 36
36 people are killed as 2, 4, 6, ..., 72. Thus the sword will now be given to the 73rd person. Now he is the first person to start in the remaining 64 people. Thus he will be the one to survive.

Solution:

20 units, 12 units & 15 units

As hard as it may sound, we have to just apply some basic algebraic equations to solve it.
Let the three sides be X, Y and Z. Now, the area of different surfaces is given:
X * Y = 240
Z * Z = 300
Y * Z = 180

Multiplying the obtained equations:
(X*Y) (X*Z) (Y*Z) = 240 * 300 * 180
X^2 * Y ^2 * Z^2 = 12960000
(X*Y*Z)^2 = 12960000
X*Y*Z = 3600

Now we know that X*Y = 240

Thus Z = 3600/240 = 15.

If we put the given values in the same manner, we get the three sides:
X = 20 units
Y = 12 units
Z = 15 units

Solution:

23rd day

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20 + 21 + 22 + 23 = 276

Thus our anniversary falls on the 23rd day of the month.

You can apply other formulas to shorten the process but that is the simplest way to do it.

Solution:

2hr 11min

Explanation:
Now this question cannot be solved unless you know a fact i.e. the hour hand is exactly on the minute mark five times every hour - on the hour, 12 minutes past hour, 24 minutes past hour, 36 minutes past hour and 48 minutes past hour.

Denoting the number of hours with X and the number of minutes as Y, let us begin with the calculation.

When we have the hour hand on the minute mark, the position of hour hand:
5X + Y/12
And the position of the minute hand = Y

When the first observation was taken,
Y = Y = 5X + Y/12 + 6
Or, 60X = 11Y - 72.

With the given facts, we know for sure that Y can only be 0, 12, 24, 36 or 48.
According to which, the possible values for X and Y can be 1 and 12 respectively. Thus the time must be 1:12 when the first observation was taken.

Similarly, in the second observation, let's make the equation:
60X = 11Y - 84
Here, the possible values for X and Y here are 3 and 24 respectively. According to this, the time is 3:24 for sure.

From 1:12 to 3:24, two hours and eleven minutes have passed.

Solution:

$6 for red sponges = 1 sponges
$87 for yellow sponges = 29 sponges
$7 for blue sponges = 70 sponges

Explanation:
a = red sponges
b = yellow sponges
c = blue sponges

Now using a simple expression:
A + b + c = 100
6a + 3b + 0.1c = 100

Now if we multiply the first equation by 6 and then 3 and then subtracting the two equations, we will be able to get two more equations:

6a + 6b + 6c = 600
3a + 3b + 3c = 300

3b = 500 - 5.9c
3a = 2.9c - 200

Now if you are wondering how will we solve three variables using just two equations, we have to tell you that a and b are non-negative integers. Thus, if 3b >/= 0, then 500 - 5.9c >/= 0.
Using this, c Also, if 3a >/= 0, then 2.9c - 200 > 0. This implies that c >/= 68.97.

But buying the blue sponges is the only way to spend in fraction thus the number of blue sponges that we buy must cost an even dollar amount. Now if you consider the expressions, there can be only two numbers we can buy between 68.97 and 84.75 that satisfies the given condition i.e. 70 and 80.

Let's say that we replace c by 80 in the last two expressions that we have formed above, a will equal to 10.67 and b will equal to 9.33. However, the values of a and b must be an integer and not fraction. Therefore, c can't be 80.

Let us now replace c with 70. After solving the expressions, we will obtain a as 1 and b as 29.

Thus here are the amounts that must be used to buy:
$6 for red sponges = 1 sponges
$87 for yellow sponges = 29 sponges
$7 for blue sponges = 70 sponges

Solution:

10 = 9.9999999... .
The proof is correct and the fact is for real. There is no error in it.

Solution:

12/27

Explanation:
This is really simple if you know the concept. You just have to increase the fraction by multiplying with a number. In that way, you will increase the numerator and you will be able to find the required fraction. Here, we are multiplying the numerator and denominator of both the numbers with 9. Now if you simplify, the number will remain the same and thus dont think we changed the fraction.

1/3 * 9/9 = 9/27
2/3 * 9/9 = 18/27

Its easy to find the one third fraction on the way now i.e. 12/27.

Solution:

If you notice the animated gif, you know that D, C and B are going to die. D dies directly after coming in contact with the object, C dies after being thrust by the spikes of the see-saw and B dies with the blue ball.

But consider the situation that the person at E stands for Ebola. Eventually A dies as well.

We thank 9gags for this awesome gif.

Solution:

Impossible

Explanation:
Let us first denote everything with variables.
d= distance to your friend's place
T = time taken to get there
t = time required to get back
R = Speed while returning back

d = 20T
T = d/20

d = Rt
t = d/R

We have made equations for T and t and now we can derive an equation for the round trip.

2d = 40(T + t)
2d = 40(d/20 + d/R)
2d = 40d(1/20 + 1/R)
1 = 20(R/20R + 20/20R)
20R = 20(R+20)
R = R + 20

Do you see the paradox here? You literally have to travel back at an infinite speed if you want to make the average speed of your trip 40 mph. The faster return speed will have lesser impact of the average speed but consider that the quicker your return trip is, the faster you will make it.

We are talking about travelling double the distance in the same time that was taken for one way trip. Thus, if you travel at infinite speed, you will be able to attain an average of 40 mph.

Solution:

This problem might seem complex but it can be solved completely using simple logics. We know that the bike is twice as fast as walking and thus the time taken by to reach hotel is equal to the time taken on foot to travel half the way. Thus, biking the whole way will be faster than taking the train if the train's speed is not infinite (not possible).