How many shirts does he need to own ?

A money-conscious student likes to wear a clean shirt every day.
He brings his shirts to the laundromat every Monday morning
and picks them up exactly one week later.
How many shirts does the budget-minded student need to own
so that he has a clean shirt to put on every day?

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EXPLANATION: 7 dirty shirts at the laundromat, 7 shirts to last until he picks them up, and the shirt he wears on Monday when he goes to the laundromat make 15 in all.