Mental strategies- 27.01.2013

Do it mentally or for practice. You may not do it in your notebook

Finding Compatibles (Review)

This strategy for addition involves looking for pairs of numbers that combine to make a sum that will be easy to work with. Some examples of common compatible numbers include 1 and 9; 40 and 60; 75 and 25 and 300 and 700.

Examples

a) For 3 + 8 + 7 + 6 + 2, think,“3 + 7 is 10, 8 + 2 is 10, so 10 + 10 + 6 is 26.”

b) For 25 + 47 + 75, think, “25 and 75 is 100, so 100 and 47 is 147.”

c) For 400 + 720 + 600, think, “400 and 600 is 1000, so the sum is 1720.”

d) For 3000 + 7000 + 2400, think, “3000 and 7000 is 10 000, so 10 000

and 2400 is 12 400.”

Practice Items

11 + 59 =                                         60 + 30 + 40 =

75 + 95 + 25 =                                 475 + 25 =

625 + 75 =                                       300 + 437 + 700 =

800 + 740 + 200 =                           900 + 100 + 485 =

400 + 1600 + 3000 =                       9000 + 3300 + 1000=

3250 + 3000 + 1750 =                     2200 + 2800 + 600 =

3000 + 300 + 700 + 2000 =             3400 + 5600 =

0.6 + 0.9 + 0.4 + 0.1 =                      0.2 + 0.4 + 0.8 +=

0.7 + 0.1 + 0.9 + 0.3 =                     0.25 + 0.50 + 0.75 =

0.4 + 0.5 + 0.6 + 0.2 + 0.5 =            0.45 + 0.63 =