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If the speed of car B is decreased by 10 kms per hour, the average speed of car B and C will be 70 km/hr. If the speed of car A is increased by 10 km/hr, the average speed of cars A and C will be 70 km/hr. The average speed of cars A and B is 70 km/hr. Find the speeds of car A and B?
1. 60, 80
2. 60, 70
3. 70, 80
4. 60, 90

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Correct Answer - Option 1 : 60, 80

Given:

If the speed of car B is decreased by 10 kms per hour, the average speed of car B and C will be 70 km/hr. If the speed of car A is increased by 10 km/hr, the average speed of cars A and C will be 70 km/hr. The average speed of cars A and B is 70 km/hr.

Formula:

Average of n numbers a1, a2 ... an = (a1 + a2 + ... an)/n

Calculation:

Let the speed of car B be b and speed of car A be a and C be c respectively.

New speed of card B = b – 10

Given,

(b – 10 + c)/2 = 70

b + c = 150       ----(1)

New speed of car A = a + 10

Given,

(a + 10 + c)/2 = 70

a + c = 130     ----(2)

Average speed of cars A and B is 70 km/hr

(a + b)/2 = 70

a + b = 140       ----3

Adding (2) and (3)

2a + b + c = 270

2a + 150 = 270

2a = 120

a = 60

b = 80

c = 70

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