Using the numbers 1 5 6 and 7 make 21
You must use all the numbers. Anyone got an idea?

I don't see anything about not using any number more than once. The only proviso is using all the numbers... Sooo...

The sum of 1 + 5 + 6 +7 = 19
Adding the number 1 twice more would = 21

you can only use them once. The closest I have is 22 by a number of routes unless I use

5+7=12
12 divide 6=2, then "add" the 1 to make 21

Though I daresay there is a more complex way involving multiplying 6 x 7 = 42
Then, 5 - 1 = 4. This is where my brain hurts as I cope with basic numbers hence the simplistic calculation above... I suspect the next thing will be something about roots that are not round and buried in the ground but are in fact square, which, like the simplistic calculation of 1 + 1, likely will equal 2. Sooo... 42 divided by 2 would = 21.

I think the first calculation was easier!