Answer:
The first option is to put $400 away initially and save $20 every month. Let she saves for x months. Equation becomes;

The second option is to put $100 away initially and save $40 every month. Let she saves for x months. Equation becomes;

Equating both as equal we will get the number of months.

=>

=>

x = 15
Hence, it will take 15 months to save the same amount.
And amount saved will be =

= $700