Question

Explain how to solve this problem by writing down the steps. The grass on the field erodes by -1 1/2% every year. If the total length of grass on the field was 220 feet long 3 years ago, how long is the length of grass on the field now?

Answer 1

Explanation:

Since the grass on the field erodes by -1 1/2% every year, it means that the rate of erosion is exponential. It is therefore eroding in geometric progression. We would apply the formula for determining the sum of n terms of a geometric sequence which is expressed as

Sn = a(1 - r^n)/(1 - r)

Where

n represents the number of terms(number of years) in the sequence.

a represents the first term in the sequence(the length 3 years ago)

r represents the common ratio(rate of erosion)

From the information given,

a = 220 feet

r = - 1.5/100 = - 0.015

n = 3

Therefore, the length of the field after 3 years, S3(length of the field now) would be

Sn = a(1 - r^n)/(1 - r)

S3 = 220(1 - (- 0.015^3)/(1 - - 0.015)

S3 = 220.0007425/1.015

S3 = 216.7495 feet