If anyone can do it please show me how !

Gina and Hayen take out a loan of $70 000 over 25 years at 8% p.a interest compunding monthly. Their repayment is $540 per month. After 5 years of repaying the loan they make a lump sum payment of $15 000.

How long will it now take to repay the loan?

The answer is 16 years and 10 months

Anyone who can work this one out is really smart or I am just really dumb!

thanks,

sez