Question 377467
Let r = speed of current.
Then 15 - r = clark's speed against the river,
15 + r = clark's speed with the current.  Then
{{{140/(15+r) = 35/(15-r)}}}, using the formula T = D/R.
140(15-r) = 35(15 + r),
2100-140r = 525 + 35r,
1575 = 175r,
r = 9 km/hr, the speed of the river.