From basic physics, distance = rate * time. Set up two equations:
Upstream: rate = 10 - r, time = t + 1, distance = 24 miles
Downstream: rate = 10 + r, time = t, distance = 24 miles
24 = (10 - r) * (t + 1)
24 = (10 + r) * t
Solve the system by substitution:
(10+r) = 24 / t
r = (24/t) - 10
24...