NCERT Class 6 Solutions: Whole Numbers (Chapter 2) Exercise 2.2–Part 1
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Q1 Find the sum by suitable rearrangement:
Solution:
The idea is to pair the numbers so that their least significant (rightmost) digits add to either 5 or better still to 10. For example when asked to add 11 + 12 + 19. First do 11 + 19 = 30 (notice how the total has 0 in rightmost digit). Now it is easy to 30 + 12 = 42.

Rearrange the sum,

Q2 Find the product by suitable arrangement
Solution:
The idea is to pair the numbers so that a number ending in 5 is multiplied with an even number. This would produce 0 in the least significant. For example when asked to calculate . First do = 180 (notice how the total has 0 in rightmost digit). Now it is easy to do = 1260 (we could ignore the zero, then multiply and then add back the zero).


Note here we choose to multiply 4 with 25 (even though 166 was also even), this was because we got 100 makes it very easy to multiply further.

Here 8 is even.

Here 16 is even.

Here 60 is the even number.

We know that product of 8 and 125 is 1000 and so we make a pair out of them. We also know that 4 multiplied with 25 would be 100 so product of 40 and 25 would also be 1000.
Q3 Find the value of the following:
Solution:
You can distribute multiplication over addition and subtraction: