1 book is 17.55
2 books is 16.70
3 books is 15.85
4 books is 15
5 books is 4.15
6 books is 4.30
7 books is (3.55)
Hopes this helps:
Answer: x = -12
How I know I use this app called Cymath.
3x - y = 10 . . . (1)
-x + y = 12 . . . (2)
(1) + (2) => 2x = 22 => x = 22/2 = 11
From 2, -11 + y = 12 => y = 12 + 11 = 23
Faucet one fills up

of the bathtub in one minute and faucet two fills up

of the bathtub in one minute. If both are one then they will fill up

So, 1/6 of the bathtub in one minute.