A restaurant customer left $3.50 as a tip. The tax on the meal was 7% and the tip was 20% of the cost including tax.

17.50

Step-by-step explanation:

you really don't need to know the tax amount.

20% of x = 3.50

(20/100) x = 3.50  Divide top and bottom of the left number by 20

(20÷20) / (100÷20)  *  x = 3.50

(1/5) x = 3.50           Multiply both sides by 5  

x = 3.50 * 5

x = 17.50

The meal cost 17.50

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