Solution:

Neither, the accountant was his sister.

Solution:

Because, people turn them on.

Solution:

Just once, because after you subtract anything from it, it's not 100 anymore.

Solution:

The letter 'n'.

wiNdow - widow
door - doNor

Solution:

252

Explanation:
A unique pattern has been made into use in this series.
All numbers are (n^3 + n^2) where n is 1, 2, 3 and so on.

In first number, n = 1
1^3 + 1^2 = 1 + 1 = 2

In second number, n = 2
2^3 + 2^2 = 8 + 4 = 12

In third number, n = 3
3^3 + 3^2 = 27 + 9 = 36

In fourth number, n = 4
4^3 + 4^2 = 64 + 16 = 80

In fifth number, n = 5
5^3 + 5^2 = 125 + 25 = 150

Similarly for the sixth number, n = 6
6^3 + 6^2 = 216 + 36 = 252.

Solution:

All

Explanation:
The phrase 'all but five died' means that all except five died. This means that only five are left alive.

Solution:

There are 99 strawberry toffees in that jar.

We know for sure that there is at least one orange toffee in that jar. Now the question tells us that on picking two random toffees, one must be strawberry. This can only be possible if all the other 99 toffees are strawberry flavored.

Solution:

end up behind bars

Check that the word "END" is spelled in an upward direction behind three occurrences of the word "BAR".

Solution:

T = 1
W = 3
O = 8
H = 9
R = 0
E = 4

For the full multiplication, see the figure.

Solution:

First Person: 3 full bottles, 1 half empty bottle and 3 empty bottles.

Second Person: 3 full bottles, 1 half empty bottle and 3 empty bottles.

Third Person: 1 full bottle, 5 half empty bottles and 1 empty bottle.