24. (A) 1.6
23. (A) 4
Amy's distance is proportional to the time spent riding, so in 12/30 of the time (12 minutes out of 30 minutes), she will ride 12/30 of the distance:
... (12/30)ยท(4 mi) = 48/30 mi = 1.6 mi
Sally's speed is (66 mi)/(3 h) = 22 mi/h. Molly's speed is (72 mi)/(4 h) = 18 mi/h.
The difference between their speeds is (22 -18) mi/h. = 4 mi/h.