Question

1. The monthly salary of a person was Rs 3200 for each of the first three years
He then got annual increment of Rs400 per month for each of following
successive 12 years. His salary remained stationary till retirement when he
found that his average monthly salary during the service period was Rs6980
Find the period of his service​

Answer 1

Answer:

40 years

Step-by-step explanation:

Monthly salary for 1st three year = Rs 3200

For 12 Years his salary inreased Rs 400 per month

4th Years - 1st year increment = 400

5th year - 2ns year increment = 400 + 400 = 800

15th Year - 12th incerement = 4800

salary after 15 th years = 3200 + 4800 = 8000 monthly

Monthly total salry in 15 years

= 3200 * 15 + ( 400 + 400*2 +.................................... + 400*12)

= 3200 * 15 + 400 * 12 * 13/2

= 3200 * 15 + 400*78

= 400 * ( 120  + 78)

= 400 * 198

let say n more years he worked

then 400 * 198   + 8000*n = 6980 * ( 15 + n)

=> 40 * 198   + 800*n = 698 * ( 15 + n)

=> 102n = 698 * 15 - 40*198

=> 102n =  2500

=> n = 25

Period of service = 15+25 =40 years