WORD PROBLEM HELP!!!

Question

I am so confused!!! Please help with these two.

The curren in a stream moves at a speed of 4 mph. A boat travels 5 mi upstream and 13 mi downstream in total time of 2 hr. What is the speed of the boat in still wanter??

 

here is the second one. I don't even know where to start on this one.

 

A tablecloth measures 96in by 72in. It is laid on a tabletop with an area of 5040in², and hangs over the edge by the same amount on all sides. By how many inches does the cloth hang over the edge??

 

Please help!!!

Answer 1

1) Let u = the speed of the boat in still water. So, the speed in upstream is u-4, and the speed in downstream is u+4.

Balance by the total time,

5/(u-4) + 13/(u+4) = 2

Multiply both sides by (u-4)(u+4),

5(u+4) + 13(u-4) = 2(u² - 16)

Simplify,

u² - 9u = 0

Since u can not be zero, divide both sides by u and solve for u,

u = 9 mph <==Answer

 

2) Let x be the length hanging over the edge.

(96-2x)(72-2x) = 5040

/4, (x-48)(x-36) = 1260

x² - 84x + 468 = 0

(x-6)(x-78) = 0

Answer: x = 6 in

Attn: x = 78 is not a solution.