Olivium bought 10 colouring books of the same kind during the Lunar New Year Sale. Fanny bought 6 of the same colouring books. Fanny also bought 2 stickers at $3.70 each. Altogether, she spent $1.80 more than Olivium. What was the amount Fanny spent?
Amount that Fanny spent on 2 stickers
= 2 x 3.70
= $7.40

Let the cost of each colouring book be 1 u. 

Amount that Olivium spent = 10 u

Amount that Fanny spent =  6 u + 7.40

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

10 u + 1.80 = 6 u + 7.40
10 u - 6 u = 7.40 - 1.80
4 u = 5.60

1 u = 5.60 ÷ 4 = 1.40

Cost of each colouring book
= 1 u
= $1.40

Amount that Fanny spent
= 6 x 1.40 + 7.40
= 8.40 + 7.40
= $15.80

Answer(s): $15.80