Mental skills

Chapter 8 Mental skills

to go through and to review the concepts taught within the chapter.

You have learnt to add and subtract pairs of single- and two-digit number mentally by

3 strategies

• making tens

• adding or subtracting near multiples of 10 and adjusting

• regrouping into tens and units

Making tens

• 58 is near to 60.

• So, we can make 60 first.

• 58+8 is the same as

• 58+2 +6 or

• 60+6

• 60+6=66

Adding or subtracting near multiples of 10 and adjusting.

• 18 and 2 make 20.

• So, adding 18 is the same as adding 20 and then subtracting 2.

• We add 20 to 44.

• 44 + 20=64

• Then we subtract 2 from the result.

• So, 44+18=62

• Remember! When we add a 20, the digit in the tens place increases by 2.

Regrouping into tens and units

• 9 and 1 make 10.

• So, subtracting 9 is the same as subtracting 10 and then adding 1.

• We subtract 10 from 25.

• 25-10= 15

• Then, we add 1 to the result.

• 15 +1=16

• So, 25-9=16 . Remember! When we subtract a 10, the digit in the tens place decreases by 1.

You have learnt to add three small numbers by first making 20.

You know to find doubles and halves of numbers mentally by regrouping the numbers into tens and units, finding their doubles or halves separately and then adding the results.

You know to find pairs of multiples of 10 that make 100.

Now, let's apply what you know. ,Let's do Putting it together 1 and 2.