Natalie bought 8 colouring books of the same kind during the Chinese New Year Sale. Esther bought 6 of the same colouring books. Esther also bought 4 marbles at $3.80 each. Altogether, she spent $1.80 more than Natalie. What was the amount Esther spent?
Amount that Esther spent on 4 marbles
= 4 x 3.80
= $15.20

Let the cost of each colouring book be 1 u. 

Amount that Natalie spent = 8 u

Amount that Esther spent =  6 u + 15.20

Esther spent $1.80 more than Natalie.  If Natalie spent another $1.80, she would spend the same amount as Esther.

8 u + 1.80 = 6 u + 15.20
8 u - 6 u = 15.20 - 1.80
2 u = 13.40

1 u = 13.40 ÷ 2 = 6.70

Cost of each colouring book
= 1 u
= $6.70

Amount that Esther spent
= 6 x 6.70 + 15.20
= 40.20 + 15.20
= $55.40

Answer(s): $55.40