Jade bought 12 textbooks of the same kind during the Easter Sale. Zara bought 4 of the same textbooks. Zara also bought 5 marbles at $2.50 each. Altogether, she spent $2.10 more than Jade. What was the amount Zara spent?
Amount that Zara spent on 5 marbles
= 5 x 2.50
= $12.50

Let the cost of each textbook be 1 u. 

Amount that Jade spent = 12 u

Amount that Zara spent =  4 u + 12.50

Zara spent $2.10 more than Jade.  If Jade spent another $2.10, she would spend the same amount as Zara.

12 u + 2.10 = 4 u + 12.50
12 u - 4 u = 12.50 - 2.10
8 u = 10.40

1 u = 10.40 ÷ 8 = 1.30

Cost of each textbook
= 1 u
= $1.30

Amount that Zara spent
= 4 x 1.30 + 12.50
= 5.20 + 12.50
= $17.70

Answer(s): $17.70