Marsha saves $92 during her first month at a new job...

Question

If she increases the amount she saves by 20% each month, how much will she have saved after 3 months?

 

a) $312.80

b) $334.88

Answer 1

Technically, this is a geometric sequence:

 

92, 92(1.2), 92(1.2)(1.2), ...

 

 

 

You could use the formula for finding the sum of the first n terms of a geometric sequence:

 

S_n = a₁(1 - rⁿ) / (1 - r), where n = term you want to find (3), r = common ratio (1.2), and a₁ = first term (92)

S₃ = 92 (1 - (1.2)³) / (1 - 1.2)

S₃ = 92 (-0.728) / (-0.2)

S₃ = 92(3.64)

S₃ = $334.88

 

 

 

Or, since we're only interested in three terms, you could solve this by brute force:

 

Month 1: $92

Month 2: 92(1.2) = $110.40

Month 3: 110.4(1.2) = $132.48

Total: $334.88