Question

Jack spent $21.70 on 6 notebooks and 7 pencils and has some money left. To buy another notebook he will be short of $0.30. To buy one more pencil, he would have $0.50 left.

How much does a notebook cost?
6n+7p = 2170
If he had x cents left, then
7n+7p = 2170+x+30
6n+8p = 2170+x-50

x = 180
n = 210
p = 130

So, a notebook costs $2.10

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