1. 10% of what number is 80?
2. 33.333% of what number is 21?

To get the result we can use proportion
10% 80
100% x
Crossmultiply now
10*x=100*80
10x=8000     /:10
x=800 - its the answer

33.333% 21
100% x
crossmultiply
33.33x=21*100
33.33x=2100         /:33.33
x=63 - its the answeer