Question

What is the sum of first 20 multiples of 2?

• A. $225$
• B. $420$
• C. $210$
• D. $217$

Answer 1

The correct option is B $420$

The sum of first n terms of arithmetic series formula can be written as,

$S_n=\frac{n}{2}[2a+(n-1\left)d\right]$

Where n = number of terms $\Rightarrow$ 20

a = first odd number $\Rightarrow$ 2

d = common difference of A.P. $\Rightarrow$ 2

Apply the given data in the formula,

$S_{20}=\frac{20}{2}[2\times 2+(20-1\left)2\right]$

$=10[4+(19\left)2\right]$

$=10[4+38]$

$=10\left[42\right]$

$=420$

So, the sum of first 20 multiples of 2 is 420.