Answer:
Part A: If he's buying 3 pens he'll have $4.54 left, if he's buying 4, he'll have $3.05 left. I don't really understand what you mean with Part B but I hope this answer and explanation for Part A helps you :D
Explanation:
3 Pens situation:
12.50 - 3.49 = 9.01
1.49 (The price of a single pen) x 3= 4.47
9.01- 4.47= 4.54.
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4 Pens situation:
12.50 - 3.49 = 9.01
1.49 (The price of a single pen) x 4= 5.96
9.01- 5.96= 3.05.