Question

84 is 75% of what number?

Answer 1

100% = 1
75% = 0.75 = 3/4

3/4x = 84     | *4
3x = 336      |  /3
x = 112 

Or you can do like this:
3/4x = 84     |    /3

1/4x = 28

3/4x + 1/4x = 84 + 24
4/4x = 112
x = 112 

Answer 2

112
x * 75/100=84 
75x/100=84 
75x=8400 
x=112