Bunuel
Everyday I cover the distance from my home and office at a usual speed and take a certain time at the usual speed. When I increase my usual speed by 5 kmph, I take 10 minutes less than usual. If I reduce my usual speed by 5 kmph, I take 15 minutes more than usual. Find the distance from home to office.
A. 25 km
B. 30 km
C. 40 km
D. 45 km
E. 50 km
Noting that 10 minutes = ⅙ hour and 15 minutes = ¼ hour, we can create the equations:
rt = d
(r + 5)(t - 1/6) = d
and
(r - 5)(t + 1/4) = d
Equating the first and second equations, we have:
(r + 5)(t - 1/6) = rt
rt - r/6 + 5t - 5/6 = rt
-r/6 + 5t - 5/6 = 0
Multiplying the above equation by -6, we have:
r - 30t + 5 = 0 → Eq. I
Similarly, equating the first and third equations, we have:
(r - 5)(t + 1/4) = rt
rt + r/4 - 5t - 5/4 = rt
r/4 - 5t - 5/4 = 0
Multiplying the above equation by 4, we have:
r - 20t - 5 = 0 → Eq. II
Subtracting Eq. I from Eq. II, we have:
10t - 10 = 0
10t = 10
t = 1
Substituting 1 for t in Eq. II, we have:
r - 20(1) - 5 = 0
r = 25
So d = rt = 25(1) = 25.
Answer: A