Question

2/3 of a box of buttons are black, 1/12 of them are green and the remainder are blue. If there are 21 blue buttons, how many buttons are there altogether?

Answer 1

2/3 means 8/12 so 
8/12+1/12=9/12
remaing12/12 -9/12=3/12
 3/12=21 then,
12/12=84