Rick has already told you how to solve the problem. I will point
out that there is more than one solution to this poorly specified
problem. My principal objection to it, however, is that it is a
*stupid* problem. What is the point of doing this calculation?
I think that good problems in physics should motivate students to
solve them because the result will be intrinsically interesting.
This one isn't. The result I fear most is that the student will
go away from the only physics course she takes thinking that this
is what physics is.