3-42 Mental Math Methods, Subtraction

When you subtract numbers in your head it is called mental math.

To make mental subtracting easier, when you have to subtract a number that ends in a 9 from another number, subtract 10 instead and then add 1 to the answer when you finish.  It's easier to subtract an even number like 10, 20 or 30 and so on.

Here's an example: 34 - 9 =

Mentally think: 34 - 10 = 24 and 24 + 1 = 25

So using mental math you now know that 34 - 9 = 25

Here's another example: 47 - 19 =

Mentally think: 47 - 20 = 27 and 27 + 1 = 28

So using mental math you now know that 47 - 19 = 28

Practice subtracting numbers that end in a 9 using mental math.

54 - 19 = 54 - 20 = +1 =

32 - 9 = 32 - 10 = + 1 =

28 - 9 = 28 - 10 = + 1 =

66 - 29 = 66 - 30 = + 1 =

47 - 19 = 47 - 20 = + 1 =

85 - 49 = - = 35 + 1 = 36

35 - 19 = - = 15 + 1 = 16

42 - 9 = - = 32 + 1 = 33

74 - 49 = - = 24 + 1 = 25

58 - 9 = -  = 48 + 1 = 49

Another way to use mental math when subtracting numbers in the hundreds is to think of an even hundred number.

Here's an example: 326 - 198 =

Mentally think: 198 rounds to 200, so subtract 200 and then add 2.

So using mental math 326 - 200 = 126 and 126 + 2 = 128

326 - 200 = 128

Here's another example: 316 - 196 =

Mentally think: 196 rounds to 200, so subtract 200 and then add 4.

So using mental math 316 - 200 = 116 and 116 + 4 = 120

116 + 4 = 120

Practice subtracting numbers in the hundreds using mental math.

400 - 196 = 400 - 200 = + 4 =

500 - 192 = 500 - 200 = + 8 =

200 - 29 = 200 - 30 = + 1 =

135 - 27 = 135 - 30 = + 3 =

316 - 198 = 316 - = + 2 = 118

212 - 37 = 212 - 40 = + 3 =

169 - 98 = 169 - 100 = + 2 =

712 - 193 = 712 - 200 = + 7 = 519