Learn how to apply the fundamental rule of proportions just follow simple few steps.
First step is take two fractions and equate them. For example (Top/Bottom)=(Top/Bottom).
Second step is to take left hand side fraction. From the top, multiply with bottom of right hand side and on the left hand side bottom, multiply with right hand side top and equate them.
Third step is calculate the value.
For example X/7=8/28.
second step is x=7*8/28.
Third step is get value of X=2.