EXPLANATION.
First we must analyze the data that the exercise gives us, to start the grandmother actually gives 5 per month but gave an initial of 1000,
in the end you have 5715.
What we must do is subtract 5715 from the 1000, then the result is divided by 5 and gives us the total number of months.
The exercise is as follows:
It takes 943 months to complete what remains for the fund.