All you do is use the a+b=c formula lets plug it in
15+b=35
15+20=35
35=35
The answer is 20
If we factorize the expression

We will get

Hope I helped!
The answer to your problem would be 400 because to find the answer you need to multiply 120 by .3 to get the answer because 3 % means per 100 and that is why you make it .3 hope i help.
The answer is in the thousands group