Question

The sum of four consecutive numbers is $50$. Find the first number

• A. $16$
• B. $18$
• C. $21$
• D. $20$

Answer 1

Answer: (C)

$21$

4 consecutive numbers form an arithmetic progression with difference 1.
$n=4,s_4=50,d=1$
we know the formula, $S_n=\frac{n}{2}[2a+(n-1\left)d\right]$
$50=\frac{4}{2}[2\times a+(4-1\left)1\right]$
$50=2[2a+3]$
$\frac{50}{2}=2a+3$
$45-3=2a$
$\frac{42}{2}=a$
$a=21$
$\therefore$ The first number is 21