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

Question 439228: In a 16 km cross-country ski race Brett skied the first 7 km at a constant speed and then increased his speed by 3 km/h for the last 9 km. If he finished the entire race in 1 hour and 20 minutes, what was his speed for the first 7 km?
my teacher gave me the answer 10.5 km/hr but i cant set it up right. i know that d = r X t but for some reason when i set up the problem i get a different answer everytime and its never the right one. thanks for your help!
Answer by jorel1380(3719) (Show Source): You can put this solution on YOUR website!
7/x+9/(x+3)=1 1/3
7/x+9/(x+3)=4/3 hrs.
Multiply all by 3(x)(x+3):
7x3x(x+3)+9x3xX=4(x)(x+3)
21x+63+27x=4x2+12x
0=4x2-36x-63
0=(2x+3)(2x-21)
x=-3/2 or 21/2
Throwing out the negative answer, we get Brett's initial speed to be 10.5 kph. Your teacher is actually right!!!!!
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