IBPS SO Prelims Exam Model Paper 3

Given:Initial speed of vehicle = 42 km/h
Increment of speed every hour = 6 km/h
Distance between the points A and B = 420 km
Formula used:
Sn = (

n

2

){2a + (n – 1)d}
Sn = Sum of terms in arithmetic progression
n = Number of terms
a = First term
d = Difference
Calculation:
Sn = 420
a = 42
d = 6
420 = (

n

2

){2 × 42 + (n – 1) × 6
⇒ 840 = n{84 + 6n – 6}
⇒ 840 = n{78 + 6n}
⇒ 840 = 78n + 6(n)2
⇒ 6n2 + 78n – 840 = 0
⇒ n2 + 13n – 140 = 0
By factorizing we get,
⇒ n = 7
∴ The time taken by the vehicle to complete the distance is 7 h.