Wade can paddle his kayak 4 miles per hour in still water. It takes him as long to paddle 5 miles upstream as it takes him to travel 15 miles downstream. Determine the speed of the river's current. Enter an equation with the following properties: 1. It uses the variable c to represent the speed of the river's current in miles per hour. 2. It uses the information as it is given above. 3. It can be solved to answer the question.

Equation: How fast is the river's current? miles per hour