A student walks from his house at a speed of 21/2 km per hour and reaches his school 6 minutes late. The next day he increases his speed by 1 km per hours and reaches 6 minutes before school time. How far is the school from his house ?

Answer: Option B

Difference between his reaching time :
= (14 - 10) hrs
= 4 hrs
= 4 hrs → 6m + 6m
(late + before)
= 4 hrs → 12 minutes
= 1 unit = $$\frac{12}{4\times 60}$$   km
($$\because$$ 1 m = 60 seconds)
1 unit = $$\frac{1}{20}$$ km
Then, 35 units :
= 35 × $$\frac{1}{20}$$ km
= $$\frac{7}{4}$$ km
Then the distance between his house and school is = $$\frac{7}{4}$$ km