Walking at 3/5 of his usual speed, a man is 4 hours too Late | Find his usual time | Speed Distance and Time

Walking at 3/5 of his usual speed,  a man is 4 hours too late.  Find his usual time.

Suppose the usual speed of the man be x km/hr .
Let distance travelled be D km
Therefore usual Time taken = D / x
.’. walking speed = 3/5. x
Let distance travelled be D km
we can write
D/(3/5 . x)- D/x = 4
Or D((5 / 3. x) – 1/x) = 4
Or D/x ((5 / 3 ) – 1) = 4
Or D/x (2 / 3) = 4
But D/x = Usual Time taken by the man .
T= 12/2
or T= 6 hours . Answer