"Junior's boat will go 15 miles per hour in still water. If he can go 12 miles downstream in the same amount of time as it takes to go 9 miles upstream then what is the speed of the current?"
distance = rate X time ... d = r*t we know the speed of the boat in still water 15 mph
we know he can go 12 miles downstream in the same time he can go 9 miles upstream-- so times are equal --- so since time is equal we can set the distances and rates equal to one another...
d1/r1 = d2/r2
The rate downstream will be 15 + x and upstream will be 15 - x so we have:
12/(15+x) = 9 /(15-x)
cross multiply and you get 12(15-x) = 9 (15+x)
180 - 12x = 135 + 9x
45 = 21x
x = 45/21 = 15/7 This is the answer the current is 15/7 or 2 and 1/7ths mph..
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The rate downstream will be 15 + x and upstream will be 15 - x right?
ReplyDeleteif im not mistaken it will be
12/(15+x)=9/(15-x) coz the 12 is the downstream and the 9 is the upstream
and if there's division outside the parenthesis it must be cross multiply so;
12/(15+x)=9/(15-x) 12/(15+x)=9/(15-x)
|_____x______| |_x__|
135+12x=180-9x
45=21x
=2.14mph
Ok the exact same solution is above but good job if you were covering the screen when you solved that.
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