In Chad's reading class, all the students are reading the same book. The school bought each student a book at $7 per book. If there are 27 students in Chad's class, which of the following expressions could not be used to calculate the total cost?

a. 7 (30) - 7 (7)

b. 7 (20) + 7 (7)

c. 7 (30) - 7 (3)

It would be A. 7 (30) - 7 (7)

a. 7 (30) - 7 (7)

Step-by-step explanation:

Given,

The total number of students = 27,

∵ Every student gets a book of $ 7,

So, the total cost (in dollars) = price of each book * total student

= 7 * 27

= 7 (27)

∵ 7 (30) - 7 (7) = 7 (30-7) = 7 (23) ≠ 7 (27)

7 (20) + 7 (7) = 7 (20+7) = 7 (27)

7 (30) - 7 (3) = 7 (30-3) = 7 (27)

Hence, option 'a' is correct.