The northbound biker travels (4 mph)(4 hr) = 16 mi, and
the eastbound biker travels (3 mph)(4 hr) = 12 mi.
They travel at right angles to one another. Thus, the Pythagorean Theorem applies here. The distance between the two bikers is d = √( [12 mi]^2 + [16 mi]^2 ), or √(144 + 256) mi, or √400 mi, which works out to 20 miles.
