Walking 3 of my usual speed, a late 4 is marked on my cards

Walking 3 of my usual speed, a late
4
is marked on my cards by 10 minutes. Find my usual time.

Since I walk at 3/4 of my usual speed the time taken is 4/3 of my usual time.
[∵ the speed and time are in the inverse ratio]

∴	4	of usual time
	3

= Usual time + Time I reach late

∴	1	of usual time
	3

= 10 minutes
∴ Usual time
= 10 × 3 = 30 minutes.