Try to make all the numbers from 1 to 10 using exactly four fours and any number of the operators + – x /
e.g. you can make 7 by doing 4 + 4 + 4/4
Can you make the rest?
If you are also allowed to use square root, factorial (!) and double factorial (!!) you can make the numbers 11 – 100.
e.g. 4! is 4x3x2x1 so you could make 28 by doing 4! + 4 + 4 – 4
Double factorial multiplies even or odd numbers like this:
4!! = 4 x 2
8!! = 8 x 6 x 4 x2
7!!= 7 x 5 x 3 x 1
Here is a full list of solutions if you get stuck, and watch the amazing video by Alex Bellos showing a solution for any number if you are allowed to use logarithms.