Question

7. Look at the number sentence below.

1+2+3+4 +-----------+ 80 = 3240
71 +72+73+ ---------+ 80 = 755
What is the sum of numbers from 1 to 70?
a)3240-10
b) 3240-80
c) 3240 +755
d) 3240-755

Answer 1

Answer: 3240-755

Explanation:

We know that:

`(1+2+\cdots+80)-(71+72+\cdots+80)=1+2+\cdots+70`

Thus,
`1+2+\cdots+70=\boxed{3240-755}`