Question

Find the sum of integers which are divisible by 5 from 50 to 85?

• A. 540
• B. 550
• C. 560
• D. 600

Answer 1

The correct option is A 540

Sum of integers which are divisible by 5 from 50 to 85
= (Sum of integers which are divisible by 5 from 1 to 85) − (Sum of integers which are divisible by 5 from 1 to 49)
= (5 + 10 + .... + 85) − (5 + 10 + .... + 45)
= 5(1 + 2 + .... + 17) − 5(1 + 2 + .... + 9)
= $5\times \left(\frac{17\times 18}{2}\right)$ - $5\times \left(\frac{9\times 10}{2}\right)$
[$\because$ Sum of first n natural numbers = $\frac{n(n+1)}{n}$]
= (5 × 9 × 17) − (5 × 9 × 5)
= 5 × 9 × (17 − 5)
= 5 × 9 × 12
= 540