Numbers from 11 to 20 are made of one ten and some ones. In addition and subtraction without regrouping across ten, we only add or subtract the ones. The ten stays the same.
12+3=15: 1 ten, ones 2+3=5
17−4=13: 1 ten, ones 7−4=3
You cannot subtract more ones than you have!
Look at the number line
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