Maths problem: how to do, thanks?

Let Mary's speed be X km/hr, Amy's speed by 2X/3 km/hr.
in 24 minutes, the total distance they have walked is 2km.
24mins =0.4hr.
0.4X +0.4(2X/3) =2
...... => X=3
Mary's speed is 3km/hr.
Amy's speed is 3(2/3) = 2km/hr