SOLUTION: If I run my first 60 km lap at a rate 25% faster than my usual 12 km/hr, and my second 60 km lap at a rate ___% slower than 12 km/hr, then my overall average speed will be my usual
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Question 241558: If I run my first 60 km lap at a rate 25% faster than my usual 12 km/hr, and my second 60 km lap at a rate ___% slower than 12 km/hr, then my overall average speed will be my usual speed
A) 50/3 B) 20 C) 25 D) 250/3 Answer by Theo(13342) (Show Source):
You can put this solution on YOUR website! Usual speed is 12 km/hr
25% faster = 12 + .25*12 = 12*(1+.25) = 12*(1.25) = 15 km/hr
25% slower = 12 - x*12 = 12*(1-x) km/hr
first 60 km lap is done at 15 km/hr
second 60 km lap is done at 12*(1-x) km/hr
overall average is 12km /hr
15 * 60 + 12*(1-x) * 60 = 120 * 12
900 + (12-12x) * 60 = 1440
900 + 720 - 720*x = 1440
add 720*x to both sides of the equation and subtract 1440 from both sides to get: