Mark wants to improve his free throw percentage in basketball. he has shot 1,250 free throws over the last 20 days. he shot either 50 or 100 free throws a day. during the 20 day period, how many times did mark shoot 100 free throws for the day?

Let's define variables:
 x = number of times he threw 50 free throws per day.
 y = number of times he threw 100 free throws per day.
 We write the system of equations that adapts to the problem:
 x + y = 20
 50x + 100y = 1250
 The solution to the system is:
 x = 15
 y = 5
 Note: see attached image.
 
 Mark did shoot 100 free throws for the day about:
 y = 5 times.