speed

[spoonthrower]

A golfer rides in a golf cart at a speed of 3.10 m/s for 21.0 s. She then gets out of the cart and starts walking at an average speed of 1.20 m/s. for how long (in sec) must she walk if her average speed for the entire trip, riding and walking, is 1.90 m/s?

I know average speed = d/t I'm just confused b/c the distance and the time for the walking part is not given. Please help. Thanks.

This HW problem is for my college physics 101 class and nobody has any ideas? c'mon

[Rozenwyn]

Just set up an equation.
3.1 \times 21 + 1.2 \times x = 1.9 \times (21+x), i.e. The total distance travelled by travelling by the cart and by walking must be equal to the total hypothetical distance travelled if he "travelled" at a speed of 1.9 m/s on whatever.
Solve for x.