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Number Bonds asks the question the other way round: instead of 7 + 3, you get 7 + ? = 10 and have to supply the missing part. Bonds to 10, to 20 and to 100 are the facts that make mental addition and subtraction fluent, because almost every larger sum is solved by bridging through a round number. This drill runs for sixty seconds and mixes the bonds so you are recalling them rather than working down a list.
Choose bonds to 10, 20 or 100 and press Start.
Each question shows one part and the whole — type the missing part.
A correct answer advances immediately; the clock keeps running.
Skip anything you have to count out, and come back to it next run.
Your score is the number of bonds completed in the minute.
Instant recall of complements — the pairs that make a round number. These are the facts that let you add 47 + 8 by going to 50 first, and subtract 100 − 63 without borrowing. They are a small set and they pay for themselves every time you do arithmetic without a calculator.
Learn them in pairs. Once 7 + 3 = 10 is instant, 3 + 7 costs nothing extra.
Bonds to 100 are bonds to 10 with a twist: the units add to 10 and the tens add to 90. 63 needs 37, because 6 + 3 = 9 and 3 + 7 = 10.
Doubles are the anchors — 5 + 5, 10 + 10, 50 + 50. Everything near them is one or two steps away.
If you are counting on your fingers, drop to the easier level. Speed comes from recall, and counting practises counting.
A number bond is a pair of numbers that add to a given total. The bonds to 10 are 0+10, 1+9, 2+8, 3+7, 4+6 and 5+5, and the same idea extends to 20 and to 100.
They are what makes bridging work. 47 + 8 is easy if you know 47 needs 3 to reach 50, and subtraction from 100 stops needing borrowing once the bonds to 100 are automatic.
No. Adults who never got them to automatic recall do arithmetic more slowly for the rest of their lives without knowing why. The set is small enough to fix in a week of one-minute drills.
Structure, not difficulty. For 100 the units digits add to 10 and the tens digits add to 9 — so 34 pairs with 66. Once you see that, the hundred bonds take about as long to learn as the ten bonds.