SOLUTION: In a 16 km cross-country race Brett skied the first 7 km at a constant speed and then increased by 3 km/h for the last 9 km. If he finished the entire race in 1 h 20 min, what was

Question 121894This question is from textbook
: In a 16 km cross-country race Brett skied the first 7 km at a constant speed and then increased by 3 km/h for the last 9 km. If he finished the entire race in 1 h 20 min, what was his speed for the first 7 km?
I have:
d=rt
7=(x+3)t
yeah...idk.thats wrong though

This question is from textbook

Answer by scott8148(6628) (Show Source): You can put this solution on YOUR website!
try t=d/r __ let x="speed for first 7 km"

for first 7 km __ t=7/x

for last 9 km __ t=9/(x+3)

total time is 4/3, so 4/3=(7/x)+(9/(x+3))

multiplying by 3x(x+3) gives 4x^2+12x=21x+63+27x __ 4x^2-36x-63=0

factoring gives (2x-21)(2x+3)=0
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