Answer: 30 books
Explanation:
Assume the books that cost K5 is x and the books that cost K7 is y.
50 books were purchased so the first formula is:
x + y = 50
They added up to K290 so the second formula is:
5x + 7y = 290
x + y = 50
5x + 7y = 290
x + y = 50
y = 50 - x
Substitute that into the second formula as y:
5x + 7(50 - x) = 290
5x + 350 - 7x = 290
5x - 7x = 290 - 350
-2x = -60
x = -60/-2
x = 30 books costing K5
y = 50 - x
= 50 - 30
= 20 books costing K7
