I don't know what grid you are talking about so I'll just show you a different method.
The question is asking you what number is 21 70% of.
So you see what 1% is by dividing 21/70= 0.3
So if 1% is 0.3, and you want to find out what 100% (actual number) is, you just multiply 1%= 0.3 by 100
= 30
Therefore, 21 is 70% of 30.
