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.

First, think of what you know. you know that the number t the end of the equation will need to be subtracted because it is the money he lost from buying the lawn mower. and your first two weeks have negative profit, so that tells you that c and d are wrong. now you can plug any of the weeks into a and b to see which one works. i'll start with plugging week 1 into a.this isn't right because it says your answer for week one should be -90.so now we need to try the same thing with b.this one got you the right answer, (sometimes if you only check with one of the weeks multiple will work so you need to check with more of the weeks, but it worked out fine here)this means that b is the correct answer.let me know if you need more clarification. i hope this : )