# 4-12 Mental Addition & Subtraction

### Lesson List

Sometimes you may need to do mental addition and subtraction.  That means you may need to add or subtract in your head.  There are several ways to do this.  Look at the example below.

58 + 29 = N

One way is to add the tens and the ones separately.

58 + 29 = (50 + 20) + (8 + 9) = 70 + 17 = 87

Another way is to add 58 and 30 and subtract 1.

58 + 29 = (58 + 30) - 1  = 88 - 1 = 87

In subtraction, you can also subtract the tens and ones separately.

58 - 29 = (58 - 20) - 9 = 38 - 9 = 29

Or, round up and subtract 30 and then add 1.

58 - 29 = (58 - 30) + 1 = 28 +1 = 29

Complete the problems below using mental math.

 Add the Tens and Ones separately. 44 + 28 = (40 + ) + (4 + 8) = 60 + = 72 36 + 26 = ( + 20) + ( + 6) = + 12 = 62 45 + 27 = ( + ) + (5 + 7) = 60 + = 57 + 28 = ( + ) + ( + ) = 70 + = Round and subtract. 44 + 28 = ( + 30) - 2 = 74 - = 72 36 + 26 = (36 + ) - 4 = 66 - = 62 45 + 27 = ( + ) - 3 = 75 - 3 = 72 57 + 28 = ( + ) - 2 = 87 - = 85 Subtract the Tens and Ones separately. 39 - 17 = (39 - ) - 7 = 29 - = 22 47 - 28 = ( - 20) - = 27 - 8 = 68 - 47 = ( - 40) - 7 = 28 - = 21 79 - 28 = (79 - ) - 8 = - 8 = Round and add. 39 - 17 = ( - 20) + 3 = 19 + = 22 47 - 28 = (47 - 30) + = + 2 = 68 - 47 = ( - 50) + 3 = + 3 = 79 - 28 = ( - ) + 2 = + 2 =