First, the replacement method (the key point is to regard the number close to the integer ten as the addition and subtraction of the integer ten)

Example 1: 46+49 (take 49 as 50- 1)

= 46+50- 1

= 96- 1

= 95

Example 2: 54-28 (consider 28 as 30-2)

= 54-30+2

= 14+2

= 16

Second, rounding method (the key point is to find a number suitable for rounding off ten)

Example 1: 25+ 16+24

= 25+( 16+24)

= 25+40

= 65

Example 2:

72- 17-23

= 72-( 17+23)

= 72-40

= 32

Third, the addition and subtraction cancellation method (it is very convenient to use when there is addition and subtraction and the addition and subtraction are very close, but you must pay attention to the operation symbols, otherwise it is easy to make mistakes. )

For example: 76- 19+ 18

=76- 1

=75

Fourth, the rule of observation

This sub-topic is very flexible, so I will give you a simple example.

What if the formula10-9+8-7+6-5+4-3+2-1is too long? Look at the red part below.

10-9+8-7+6-5+4-3+2-1Did you find a pattern?

= 1+ 1+ 1+ 1+ 1=5