Question

How many terms of the series 20 + 16 + 12+... amounts to 48?

• A. 3
• B. 5
• C. 8
• D. Both a and c

Answer 1

The correct option is D Both a and c

Best way is to go through options
20 + 16 + 12 = 48,
hence number of terms = 3
Again 20 + 16 + 12 + 8 + 4 + 0 + (-4) + (-8) = 48
hence number of terms = 8
Thus choice (d) is most appropriate answer.

Alternatively
$S_n=48=\frac{n}{2}[2\times 20+(n-1\left)\right(-4\left)\right]\Rightarrow 96=40n-4n^2+4n\Rightarrow n^2-11n+24=0\Rightarrow (n-8)(n-3)=0\Rightarrow n=8orn=3$
Hence (d) is the correct answer.