Explanation:The Use the given conditions in the problem to frame equations in the unknown x and y:
Let, speed of car "I" be x km/h and speed of car "II" be y km/h
- Distance covered by car "I" in 6hours =6x
- Distance covered by car "II" in 6 hours = 6y
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Case: 1
If they move in the same direction, they meet in 6 hours, Then,
Since both the cars are travelling in the same direction, sign will be negative.
[tex]\sf\implies 6x - 6y = 120 \: \: \: \: \: \: \\ \\ \sf\implies 6(x - y) = 120 \: \: \: \: \: \\ \\ \sf\implies x - y = \frac{120}{6}... (1) \\ \\ \sf\implies x - y = 20 \: \: \: \: \: \: \: \: \: \: \: \: \\ [/tex]
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Case: 2
If they move in opposite directions, they meet in 1 hour 12 minutes. i .e
[tex] \bf\implies 1 + \frac{12 }{60} =1 + \frac{1 }{5} =\frac{ 6}{ 5} hr \\ [/tex]
Since both the cars are travelling in opposite directions, the sign will be positive.
[tex] \sf\implies \frac{6x}{5} + \frac{6y}{5} = 120 ... (2) \\ [/tex]
Find the value of one variable in terms of other variables from the first equation:
[tex] \sf\implies x=20 + y........(3) \\ [/tex]
Substitute the value of x=20+y in second equation and solve it:
[tex]\sf\implies \frac{6}{5}x + \frac{6}{5} y= 120 \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ \\\sf\implies \frac{6(20 + y)}{5} +\frac{6}{5} y = 120 \\ \\ \sf\implies \frac{120 + 6y}{5} + \frac{6y}{5}= 120 \: \: \\ \\ \sf\implies \frac{(120 + 6y + 6y)}{5} = 120 \\ \\ \sf\implies 120 + 12y = 120 \times 5 \: \: \: \: \\ \\ \sf\implies 120 + 12y = 600 \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf\implies 12y = 600 - 120 \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf\implies 12y = 480 \: \: \: \: \: \: \: \: \: \: \:\: \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf\implies y=\frac{480}{12} \: \: \: \: \: \: \: \: \: \: \:\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf\implies y = 40 \: \: \: \: \: \: \: \: \: \: \:\: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ [/tex]
Put the value of y in eq. 3 to find the value of x:
[tex]\sf\implies x = 20 + y \: \: \: \\ \\ \sf\implies x = 20 + 40 \\ \\ \sf \implies x = 60 \: \: \: \: \: \: \: \: \: \\ [/tex]
Hence, the speeds of the two cars are 60 km/h & 40 km/h.