Answer:
45 mph = x
Step-by-step explanation:
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Distance (D) = Rate (R) * Time (T)
For Car A:
D = 30T {Eqn 1}
For Car B:
The Distance D and the time T is the same as for Car A
But, unlike Car A, Car B travels at two different rates. It travels at 20mph for a time T1. Then it passes Car A and travels at a rate of x for a period of time T2.
D = (20T1) + (x * T2)
Note that T2 must equal the total time T minus T1, so
D = (20T1) + (x * (T - T1)) {Eqn2}
Substitute Eqn 1 into Enq 2 : 30T = 20T1 + xT - xT1
Rewrite as: 30T = 20T1 + x(T-T1) {Eqn 3}
At what point do the two cars pass one another?
Car A travels 30T1 = D1
Car B is traveling in the opposite direction at 20T1 = D2
D2 = D - D1 (where D is the total circular length of the track)
Substituting, 20T1 = D - 30T1. Or 50T1 = D {Eqn 4}
Now we can establish a relationship between T and T1 by substituting Eqn1 into Eqn4
D = 30T = 50T1
30T = 50T1
(30/50)T = T1
T1 = (3/5)T {Eqn 5}
Now substitute Eqn 5 into Eqn 3
30T = 20T1 + x (T-T1)
30T = 20(3/5)T+ x (T - 3/5 T)
30T = 12T + xT(2/5)
18T = (2/5)x
(5/2)18 = x
45 mph = x
Recall that x is the speed that Car B must travel after it passes Car A.
