Answer:
If you think it in this way, You will have started from- $500 and start adding 70 per month.
For plan B, you will have no negative cash and will start adding 60.
For plan A, it will take 8 months to start paying out of you pocket. You will have already payed 420 for plan B.
Like that it will take 42 months to cost the same.
Explanation: