70% is 70 out of 100. And if the player is free throwing a total of 80 times, then 80 is 100% of the free throws. Both of the values that represent 100% are the denominators in the fractions.
The equations are equal, and whatever you do to the bottom, you have to do to the top. 100 was multiplied by 4/5 to get 80. So 70 is multiplied by 4/5 to get x.