Solution:

24

Explanation:
There are a total of 24 triangles in the given figure.

To make it easy to understand, we have put numbers on each distinct triangle. So let us start counting them now.
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, (1+2), (1+2), (1+2+3), (1+2+3+4), (2+3), (2+3+4), (3+4), (5+6), (5+6+7+8), (7+8), (1+2+5+6), (3+4+7+8), (2+6+9), (3+7+10), (6+9), (7+10) and (6+7+9+10).

Solution:

Costa

Explanation:
c + 1 characters-> d
o + 2 characters-> q
s + 3 characters-> v
t + 4 characters-> x
a + 5 characters-> f

=> dqvxf = costa

Solution:

3 1 2 2 1 1
1 3 1 1 2 2 2 1

Read Line as one 1 => 1
Read Line 2 as two one => 2 1
Read Line 3 as one two and one one => 1 2 1 1
and so on.

Solution:

Old password : altered

New Password : distinct

Solution:

Here is a way to do it. We dont know if it can be done using some other arrangement or not. If you know, do send us.

4 | 3 | 8
9 | 5 | 1
2 | 7 | 6

Solution:

The stranger said that he thought the room was his. If so, he must have had the keys to the room and thus he must not have knocked.

Solution:

They used the planks in the manner that is shown in the figure.

Solution:

31

Explanation:
Ramanujan lighting the candles in binary.
11110 =30
11111 =31

Solution:

I spoke with Joe.

Explanation:
There can be only two cases. Either Zoe is a liar or Joe is a liar.

First Case
Let us assume that Zoe is a liar and Joe is a truthful person.
If I asked the question from Joe, the answer will be yes. If I asked the question from Zoe, the answer will be no. Thus in this case, I must have asked from Joe.

Second Case
Let us assume that Zoe is a truthful and Joe is a liar.
If I asked the question from Joe, the answer will be yes. If I asked the question from Zoe, the answer will be no. Thus in this case as well, I must have asked from Joe only.

Solution:

8/(1−1/5)

Solution:

This can be done if you draw a five pointed star and place the balls on the corners and intersection points. See the figure to understand better.