Answer: 6 mph
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Explanation:
distance = rate*time
90 = r*t
where r is the slower speed and t is the time it takes when going that slower speed.
If r is bumped up 24 mph faster, to r+24, then Santos takes t-12 hours to get there. The second equation is
90 = (r+24)(t-12)
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We can solve the first equation for r to get r = 90/t
Then plug this into the second equation and do a bit of algebra
90 = (r+24)(t-12)
90 = (90/t+24)*(t-12)
90 = 90 - 1080/t + 24t - 288
90t = 90t - 1080 + 24t^2 - 288t
0 = 90t - 1080 + 24t^2 - 288t-90t
0 = 24t^2 - 288t - 1080
24t^2 - 288t - 1080 = 0
If you apply the quadratic formula, then you should get the two solutions t = -3 and t = 15. Due to time constraints, I'll skip these steps.
We'll ignore the negative t value. It makes no sense to have a negative time.
So we focus on t = 15 as the only solution.
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If t = 15, then,
r = 90/t
r = 90/15
r = 6
Santos rode his bike at 6 mph at first. Going this speed means he takes 15 hours.
If he rode 24 mph faster, at 6+24 = 30 mph, then he would ride for 15-12 = 3 hours instead. Note that 90/30 = 3.
