Elementary Mathematics

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Mental Mathematics: Addition

Using Compatible Numbers

These are numbers whose sums are easy to calculate mentally

300+20=320

100+200=300

50+50=100

130+70=200

130+50+70+20+50

Trading Off

17+36

(add 3 to make a mulitiple of 10) 17+3=20

(subtract 3 to compensate for the 3 that was added) 36-3=33

(add the two numbers) 20+33=53

Breaking Up and Bridging

47+16

(add the first number to the tens in the second number) 47+10=57

(add this sum to the units in the second number) 57+6=63

Adding from the left

57+26

(add the units) 7+6=13

(add the two sums) 80+13=93

(add the tens) 50+30=80