First, we should find the total if you were to buy these 7 snacks. We can find the total by multiplying the cost of 1 snack by 7:

So the total after you buy 7 snacks would be $8.75. You were given $20. To find how much you would have left, let's subtract the amount that you spent on the 7 snacks by the amount you originally had:

So now we know that
you will have $11.25 left after you buy 7 snacks.