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Option 3 : 110 km/h
Calculation:
Let t be the usual time taken and u is the usual speed and d is the distance,
⇒ d/u = t
When speed increased by 20%
⇒ u1 = 1.2u
⇒ time = t - 0.5
d = 1.2u × (t - 0.5)
⇒ 1.2ut - 0.6u
∴ ut = 1.2ut - 0.6u
⇒ 0.2ut = 0.6u
⇒ t = 3hrs
when u2 = u + 10
⇒ time = t - 0.25 = 2.75
∴ d = (u + 10) × (2.75) = 2.75u + 27.5
⇒ 3u = 2.75u + 27.5
⇒ u = 110
car speed is 110 km/h.
When a quantity increased by m%,
New value = (1 + (m/100)) × old value
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