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
