Answer: 3 miles per hour
<u>Step-by-step explanation:</u>
Use the formula "distance (d) = rate (r) x time (t)" to create a system of equations.
Let "r" represent the rate they are rowing
Let "c" represent the current
                             time          rate      distance        <u>EQUATION</u>
Downstream:     4 hours     r + c       40 miles         4(r + c) = 40
Upstream:         10 hours    r - c        40 miles         10(r - c) = 40
Distribute, then eliminate r to solve for c:
Down: 4r + 4c = 40  →   5(4r + 4c = 40)  →   20r + 20c = 200
   Up: 10r - 10c = 40  → -2(10r - 10c = 40) → <u>-20r + 20c</u> =<u>  -80</u>
                                                                                 40c = 120
                                                                               <u> ÷40  </u>  <u>÷40 </u>
                                                                                     c =   3