Question 123240
The question says that Erza sold AT MOST 30 more than twice the number Tava sold, so the lowest number that Erza could've sold would me 1 more than twice the number Tava sold.

Let x= the number of coupon books that Tava sold
Let 2x+30= the number of coupon books that Erza sold.
(you can use 2x+1, 2x+2, 2x+3, etc...)

x+2x+30=150
3x=120
x=40

Therefore 150-40=110 

Tava sold 40 books and Erza sold 110 books.