Diners frequently add a 15% tip when charging a meal to a credit card. what is the total amount charged if the cost of the meal, without tip, is $10?

That'd be m (the cost of the meal) plus 0.15m (the 15% tip).  A shorter way to write that would be m + 0.15m.  Here, m=$10, so the meal will cost 1.15($10), or $11.50 (answer).

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