Question
A car travels from point A to point B at a speed 20%
slower than its usual speed and arrives 30 minutes late. If the distance between A and B is 150 km, what is the car's usual speed?Solution
Let the usual speed of car = 10x km/h Speed of car when it travels with 20% less speed = 80% of 10x = 8x Difference of time taken = 30 minutes 
x = 7.5 usual speed of car = 10x = 75 km/h
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