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Answer: 14.85 + c = 20</h3>
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Reason:
To compute the change, we subtract the amount William gives the cashier (the $20) and the price of the book ($14.85)
The change c is
c = 20-14.85
To rearrange this equation, we can add 14.85 to both sides to end up with 14.85 + c = 20
In other words, solving that equation in bold leads to c = 20-14.85 = 5.15 which is the change William is given.
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Extra info (optional section)
If the cashier wanted to mentally figure out the change in his/her head, then notice the jump from 85 cents to 100 cents is 15 cents. That means we go from 14.85 to 15.00
Then the jump from $15 to $20 is an extra 5 dollars. Overall, the total change given back to William is 0.15+5 = 5.15 dollars.
