Question

Solve the following: 9. The sum of a number and 1 of the number is 40. Find the number.
the answer is 30 but how?
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Answer 1

Answer:

We could just whip out a calculator but there’s a cleaver trick we can use to add them up really quickly in our heads.

1 + 9 = 10

2 + 8 = 10

3 + 7 = 10

4 + 6 = 10

which leaves us with just 5

add them up and we get 45.

We can generalise this to get the sum of all counting numbers between 1 and n.

We’re basically multiplying n+1 by half of n.

(n+1) (n/2) = n(n+1)/2

Knowing this adding up all the numbers between 1 and 100 should be child’s play and you can do it in your head far faster than you could do it on a calculator.

100(101/2) = 10100/2 = 5050.

If you’re asked to do a party trick you can remember this

i hope it's helpful

please mark me brainliest answer