Bunuel
Xavier starts from P towards Q at a speed of 30 kilometre per hour and after every 12 minutes increases his speed by 5 kilometre per hour. If the distance between P and Q is 51 kilometres, then how much time does he take to cover the distance?
A. 60 minutes
B. 72 minutes
C. 90 minutes
D. 120 minutes
E. 100 minutes
Every 12 minutes --> 12/60 = 1/5 hour Speed increases by 5 kmph
0 - 12 min, speed = 30 kmph, Distance covered = 30*1/5 = 6 miles
12 - 24 min, speed = 35 kmph, Distance covered = 35*1/5 = 7 miles
24 - 36 min, speed = 40 kmph, Distance covered = 40*1/5 = 8 miles
36 - 48 min, speed = 45 kmph, Distance covered = 45*1/5 = 9 miles
48 - 60 min, speed = 50 kmph, Distance covered = 50*1/5 = 10 miles
60 - 72 min, speed = 55 kmph, Distance covered = 55*1/5 = 11 miles
Total distance covered = 51 kms after 72 min
Alternate Method:
Distance covered in every 12 minutes will be in AP series, whose sum should be 51 miles
6, 7, 8, . . . .
--> 51 = n*(a + \(a_n\))/2 = n/2*(a + a + (n - 1)d)
--> 51 = n/2*(2*6 + (n - 1)1)
--> 102 = 12n + n^2 - n
--> n^2 + 11n - 102 = 0
--> (n + 17)(n - 6) = 0
--> n = 6
So total time = 6*12 = 72 min
IMO Option B
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