His boat is traveling 15 kph relative to the stream.

Assume the speed of the stream is "s." The travel times can be equated like this

35/(15 - s) = 140(15 + s)

Cross multiplying, we get

35*(15 + s) = 140*(15 - s)

35*15 + 35s = 140*15 - 140s (use the distributive property)

s(35 + 140) = 15(140 - 35) (add 140s - 35*15 to both sides, factor)

s = 15*105/175 = 9

Assume the speed of the stream is "s." The travel times can be equated like this

35/(15 - s) = 140(15 + s)

Cross multiplying, we get

35*(15 + s) = 140*(15 - s)

35*15 + 35s = 140*15 - 140s (use the distributive property)

s(35 + 140) = 15(140 - 35) (add 140s - 35*15 to both sides, factor)

s = 15*105/175 = 9