3/4 x =15

$$\\3/4=0.75\\ 0.75X=15\\ $divide both sides by 0.74$\\ X=15/0.75$$

3/4 = 75% 

So we need to add on 1/4 (25%) and to increase an amount.

1 + 0.25= 1.25

3/4 x =15

15*1.25=18.75

X= 18.75 

Bringing the x to the other side gives you $${\mathtt{x}} = {\frac{{\mathtt{15}}}{{\mathtt{0.75}}}}$$

15/0,75 equals 20

Ergo,  $${\mathtt{x}} = {\mathtt{20}}$$