Are you sure you wrote that equation right
which- question- do- i- answer- ? ? ?
The answer is 30 because you put 21/x= 70/100
you cross multiply 100 and 21 you get 2100 and you divide it by 70 you should get 30 cause you take out one 0 from both numbers and end up with 210 divided by 7 then you get 30
Equations -
I like to find the decimal expression of the fractions, to make this easier.

Divide each sides by 0.6

Answer: 6/7
A statement also know as an axiom which is taken to be true without proof.