Mr Lau usually drives his car from his house to his house to his girlfriend's house. His average speed is 80km/h. On a particular day when he is not feeling well, he decides to drive at a much slower speed. His average speed for that particular day is 60km/h. On that day, he arrives at his girlfriend's house 15 minutes later than usual. How far away is Mr Lau's house from his girlfriend's house?

I have no idea how to solve this question.. Dunno how to start..

how to get 60??? as in he choose from 80km/h or 60km/h izzit??
lol.. it's a 4mark question.. (abit stupid.. cuz i graduated from my O lvl lyk 2 years ago.. was a N(A) student.. den my math teacher told me there wun be any question with 3 marks n above...) suddenly c this question, i stuck... n believe me.. i was an A student in math... zzzzzzzzzzz....

will only get the ans when my SIL's teacher go tru with her in school.. it is from PCSS's N(T) math prelim 2008..