Number Decomposition Examples

Mental Math · Axiom Academy

EXAMPLE Decomposing 28 to Add 67 + 28 Mentally Breaking 28 into a friendly multiple of ten and a small adjustment makes mental addition easy — the same idea powers multiplication too. Calculate 67 + 28 mentally by decomposing 28 into a friendly multiple of ten and a small adjustment. Excellent work — you've mastered number decomposition for mental math. Remember: Friendly numbers are multiples of ten (like 30 , 50 , 100 ) — easy to add or subtract mentally. Adjust after using a friendly number: added too much? subtract the difference. Added too little? add it. For multiplication, place-value decomposition breaks a number into tens and ones, e.g. 45 = 40 + 5 . The distributive property justifies it: . Both techniques share one idea: turning one hard calculation into two easy ones. The two answers: 67+28=95 by compensation, and by place-value decomposition. Practice with different numbers to build your mental math speed — the same decomposing habit works everywhere.

This is the written version of the interactive lesson above. See the full Mental Math course.