A man can row at 15 kmph in still water. If the velocity of current is 3 kmph and it takes him 1 hour to row to a place and come back, how far is the place?

Question: A man can row at 15 kmph in still water. If the velocity of current is 3 kmph and it takes him 1 hour to row to a place and come back, how far is the place?

Solution:
Speed downstream = (15 + 3) kmph = 18 kmph
Speed upstream = (15 - 3) kmph = 12 kmph
Let the required distance be x km
Then,
(x/18) + (x/12) = 1
(2x + 3x)/36 = 1
5x/36 = 1
5x = 36
x = 36/5
x = 7.2 km