Hi Dee.

Velocity (speed) is distance divided by time. We often writen this as:

(1)

We can solve this for d:

(2)

So if we use the subscripts _C and _W to refer to the cart and walk parts and the subscript _T to refer to the total, then we can write it this way:

(3)

Now since the total distance and time are just the sums of the two parts of the trip, we can replace d_T and t_T as shown below:

(4)

Next we use equation (2) to replace d_C and d_W:

(5)

It may look rather complicated, but you know all the values for these variables except t_W. Solve for t_W which is the time the man walks, substitute the values you know and you will answer your question.

Hope this helps,
Stephen La Rocque.