Question

The sum of arithmetic progression is 20, the first term being 8 and the common difference is -2. Find the number of terms in the series.

Answer 1

If $a$ is the first term of an arithmetic progression and $d$ is the common difference, then the sum of first $n$ terms is $an+\frac{n(n-1)}{2}d$

So we have $20=8n-n(n-1)$, that is $n^2-9n+20=0$. So $n=4$ or $n=5$

Answer: $4$ or $5$