Grouping concept.
For every 5 pens sold, another 2 are given free.
The 7 pens here can be counted as 1 group.
Cost of 1 such group = $2 × 5 = $10
He wants to buy 65 pens. How many of such groups can he buy at most?
65 ÷ 7 = 9 R 2
So he can buy 9 groups (9 × 7 = 63 pens) and the remaining 2 pens are bought at individual price.
9 × $10 + 2 × $2
= $90 + $4
= $94 (answer)