I think 30c+3d would be the correct answer.
Recall your d = rt, distance = rate * time.
b = speed rate of the boat.
c = speed rate of the current.
keeping in mind that, as the boat goes Upstream, against the current, it's speed is not "b", but is really " b - c ", because the current is subtracting speed from it.
likewise, when the boat is going Downstream, because is going with the current, is really going faster at " b + c ".

what is the speed of the boat? well, 61 + c = b.
Answer: $10
Step-by-step explanation:
Let's analize the information given: We know that she had $94 to spend on 8 books and after buying these books she had $14. So, we need to calculate the amount of money she spent. Subtract $94 and $14:

Then, to calculate the cost of each book, we need to divide the "Total spent" by 8:

5x=3x+20
5x-3x=3x-3x+20
2x=20
2x/2=20/2
X=10 Hope this helped!
-Twix