Boston is 200 mi away from NY. It takes me 1 hour less to drive to Boston when I drive 10 mi faster than my normal rate. What is my normal rate?
D = RT (Distance = Rate x Time)
Normal Rate
R = rate
T = time
RT = 200
Increased Rate/less time
R + 10
T - 1
(R + 10)(T - 1) = 200
Solve these two equation simultaneously.
(R + 10)(T - 1) = 200
RT = 200
Expand the first equation and substitute RT = 200, for RT in that expansion.
Then solve for R and T (quadratic equation).