One boat travelling 15 mph goes 47 miles downstream in the same amount of time that another boat going 20 mph goes 40 miles upstream. how fast is the current in mph? (round your answer to the nearest tenth of miles per hour and enter only the numerical part

  3.9 mi/h

Step-by-step explanation:

We assume that the given speeds are the speeds of the boats relative to the water. If c is the speed of the current, we have ...

  time = distance/speed

  47/(15 +c) = 40/(20 -c)

  47(20 -c) = 40(15 +c) . . . . . . multiply by (20-c)(15+c)

  940 -600 = 40c +47c . . . . . add 47c-600

  340 = 87c . . . . . . . . . . . . . . . simplify; next divide by 87

  c = 340/87 ≈ 3.9080 . . . . mi/h

The speed of the current is about 3.9 mi/h.

step-by-step explanation:

δabc ≅ δdcb by sas because

db ≅ ac . . given (side)

bc ≅ bc . . reflexive property (side)

∠dbc ≅ ∠ acb . . alternate interior angles where transversal bc crosses parallel lines bd and ac (included angle)