How to solve this usuing equations

Question

Kathleen made 35 pies. There were 10 slices in each strawberry pie and 7 slices in each blackberry pie. If Kathleen made 290 slices of pie in total, how many of each type of pie did she make?

Answer 1

Let x = number of strawberry pie, and y = number of blackberry pie

Then x + y =35 for total pies, 

and 10x + 7y = 290  for total slices

Solve two equation for x and y

by substitution of y = 35 - x from the first equation

     into the second, and solve for x, 

     x = 15, and get y = 20

therefore, 15 strawberry pies, and 20 blackberry pies.