Vanessa bought 10 guide books of the same kind during the Lunar New Year Sale. Natalie bought 5 of the same guide books. Natalie also bought 3 buttons at $3.20 each. Altogether, she spent $2.10 more than Vanessa. What was the amount Natalie spent?
Amount that Natalie spent on 3 buttons
= 3 x 3.20
= $9.60

Let the cost of each guide book be 1 u. 

Amount that Vanessa spent = 10 u

Amount that Natalie spent =  5 u + 9.60

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

10 u + 2.10 = 5 u + 9.60
10 u - 5 u = 9.60 - 2.10
5 u = 7.50

1 u = 7.50 ÷ 5 = 1.50

Cost of each guide book
= 1 u
= $1.50

Amount that Natalie spent
= 5 x 1.50 + 9.60
= 7.50 + 9.60
= $17.10

Answer(s): $17.10