you can set it up as a proportion
is (part) %
------- ------
of( whole) percent
you fill in the spots so it looks like this. 70% is the percent, and the 210 as the of before it, so by default, the "is" becomes x.
x 70
------ ------
210 100
then you cross multiply, so 100 times x is 100x and 210 times 70 is 14700.
so, 100x=14700
then you have to get x by itself, to cancel out x you do the opposite of the opperation which in this case means to divide, so 100x/100 is x. but what you do to one side, you have to do to another, so 14700/100=147
so X=147, or 147 is 70% of 210
you can also check your answer by multiplying the "of" you get by the percent, and you should get the "is".