Question

How to do this problem?

Answer 1

Time to double = 9.23 years
Amount after 10 years = $4,238.55

Explanation:

Compound interest C I, Amount A,
Principal P, Rate of Interest R in %, Number of years N.
`A=P(1+(R/100))^N`
For the amount to get doubled of Principal,
`4000=2000(1+(7.8/100))^N`
`(107.8/100)^N=2`
`N.log(1.078)=log(2)`
`N=log(2)/log(1.078)=9.23 years`

Amount after 10 years,
`A=2000*(1+(7.8/100))^10`
`A=2000*(1.078)^10=2000*2.1193=$4,238.55`