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Labor Day Maths Puzzle

Labor Day Maths Puzzle Solution - 9 January

Manish drive to his office at 20km/hr. After reaching office, he realize that today is a holiday of 'Labor Day'.He went back at average speed of 30km/hr. Discounting the time spent in the stoppage what was his average speed of his journey ?

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Solution
24km/hr (not 25km/hr which might be guessed by many)

D : distance traveled
T1 : time of going to office
T2 : time of returning back
Y : Average Speed

D = 20 * T1
T1 = D/20

D = 30 * T2
T2 = D/30


2D = Y(T2 + T1)
2D = Y(D/20 + D/30)
2D = Y(3D/60 + 2D/60)
2D = Y(5D/60)
Y = 120D/5D

=> Y = 24