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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!!!

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1 Answer

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)


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.




u2 - 9u = 0

This is where I was messing up? How did you get that?

Use the units.  miles/mph = hours

hours going upstream + hours going downstream = 2

5(u+4) + 13(u-4) = 2(u2 - 16)

5u+20+13u-52 = 2u2 - 32

Add 32 to both sides,

18u = 2u2

/2, 9u = u2

u2 - 9u = 0