Question 4805
You would use the distance formula: d = r*t   distance = rate*time

Let d1 = the distance covered by the 1st cyclist.
Let d2 = the distance covered by the 2nd cyclist.

So, you can write: d1 = r1*t1 and d2 = r2*t2

Since the question is...at what time will they be 15 miles apart? then t1 = t2 and d1 = d2+15 miles.

Write the distance formulas in terms of t and set them = to each other:

t1 = d1/r1 = d2/r2 Substitute the d1 = d2+15 and r1 = 20 mph and r2 = 14 mph

(d2+15)/20 mph = d2/14 mph  Now solve for d2

Once you have found d2, you can get d1 (d1 = d2+15 miles)
Once you have d1 and d2, you can find t (t = d1/r1 = d2/r2)