Free interactive math game: 80 - ? = 25 — To 100. Part of the Missing Numbers (Gaps) category on MathKite.
Taken from the generator that powers this exercise. A fresh set appears every time the page is opened, and the printable worksheet holds ten of them.
Taking one number away from another.
Subtract the ones first, then the tens. If the top digit is too small, exchange one ten from the column to the left.
For 70 - 31, the ones need an exchange: 10 - 1 = 9, and 60 - 30 = 30, giving 39.
Subtracting the smaller digit from the larger out of habit. In 70 - 31 the ones column is 0 - 1, which needs an exchange rather than a flip to 1 - 0.