Line them up
10.00
- 0.55
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The number above the 5's is less than 5 so we have to borrow
10.00 becomes 9.9(10) - because we borrowed a ten the last number is 10
9.9(10)
- 0.55
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Subtract : 10 - 5 = 5, 9 - 5 = 4, 9 - 0 = 9
9.45 is the answer.