Brain Teasers and Puzzles 

Workings

Hint
The first nine prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23.

Answer
29.

The first ten prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29.

Workings

Hint
It depends on the length of the word.

Answer
2 buttons.

The number of buttons is 10, minus the length of the word.

Workings

Hint
How many walls can be built in 20 days?

Answer
4 days.

Reasoning
In 20 days, Sam can build 1 wall, and Taylor and Kelly can each build 2 walls.

So, in 20 days, 5 walls can be built.

Therefore, one wall will take 4 days to build.

Workings

Hint
Try working backwards.

Answer
148.

148 x 6 = 888.

The easiest way is to divide successive 8's by 6 until you get an integer:
  8 ÷ 6 = 1.3333
 88 ÷ 6 = 14.666
888 ÷ 6 = 148