Roberto overtakes Juanita at the rate of (7.7 mi)/(11 h) = 0.7 mi/h. This is the difference in their speeds. The sum of their speeds is (7.7 mi)/1 h) = 7.7 mi/h.
Roberto walks at the rate (7.7 + 0.7)/2 = 4.2 mi/h. Juanita walks at the rate 4.2 - 0.7 = 3.5 mi/h.
_____ In a "sum and diference" problem, one solution is half the total of the sum and difference. If we let R and J be the respective speeds of Roberto and Juanita, we have R + J = total speed R - J = difference speed Adding these two equations, we have 2R = (total speed + difference speed) R = (total speed + difference speed)2 . . . . . as computed above
