Question 74

A car starts running with the initial speed of 40 kmph, with its speed increasing every hour by 5 kmph. How many hours will it take to cover a distance of 385 km?

Solution

There is an Arithmetic Progression here,

40 + (40+5)+ (40+5+5) .....n terms = 385

So, sum of terms = 385

first term = 40 and common difference = 5

S = n/2[2a + (n-1)*d]

385 = n/2[80+5n-5)

770 = n[75+5n]

154 = n(15+n)

n = 7

So it takes 7 hours to cover a distance of 385 kms.

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