A girl lives 500 metres from school. She sets out walking at 2m/s, but when she has walked a distance of x metres her speed has dropped to (2-x/400)m/s. How long does she take to get to school.

Can you do the working-

Just if you are interested this is my working for the question-

dx/dt=(2-x/400)

dx/(2-x/400)+k=dt

-400in(2-x/400)+k=t

Is this right?

So after this do I substitute 500 into x or something to find the time- what occurs from here-

I got a really strange answer- it is an incorrect answer. Urgent!