Janet wants to calculate the time it takes to earn a certain amount of interest on a principal amount in an investment with simple interest. What
equation can she use?

The time it takes to earn an amount A with a given principal P invested for a time period t is t = (A/P - 1)/r

Step-by-step explanation:

In order to calculate the time it would take to earn a certain amount of interest on a principal amount in an investment with simple interest, we use the simple interest formula as follows;

A = P·(1  + r·t)

A = The final amount after the number of periods of investment

P = The principal or initial amount

r = The interest rate per period

t = The number of the time periods

Therefore, the time it takes to earn a certain amount of interest on a principal amount in an investment with simple interest is fount by making t the subject  of the above equation, which is given as follows;

A/P = 1 + rt

rt = A/P - 1

t = (A/P - 1)/r

Therefore, the time it takes to earn an amount A with a given principal P invested for a time period t is given by t = (A/P - 1)/r.