MahmoudFawzy
A dog, a cat, and a mouse are at point A on triangle ABC. All three simultaneously run along triangle ABC, covering AB, then BC, and finally CA.
The dog moves at speed 12 on AB, 10 on BC, and 15 on CA.
The cat moves at speed 15 on AB, 15 on BC, and 10 on CA.
The mouse moves at speed 10 on AB, 20 on BC, and 12 on CA.
If all three finish at A at the same time, what is the measure of angle ABC?
A) 30
B) 45
C) 60
D) 90
E) 120
Source :
MindYourDecisions Since time is the same, we can make three equations. (1) \(\frac{x}{15}+\frac{y}{15}+\frac{z}{10}=\frac{x}{10}+\frac{y}{20}+\frac{z}{12}\)....Multiply both sides by 60, LCM of denominators
\(4x+4y+6z=6x+3y+5z......2x=y+z\)
(2) \(\frac{x}{15}+\frac{y}{15}+\frac{z}{10}=\frac{x}{12}+\frac{y}{10}+\frac{z}{15}\).....Multiply both sides by 60, LCM of denominators
\(4x+4y+6z=5x+6y+4z......x+2y=2z\)
(3) \(\frac{x}{10}+\frac{y}{20}+\frac{z}{12}=\frac{x}{12}+\frac{y}{10}+\frac{z}{15}\).....Multiply both sides by 60, LCM of denominators
\(6x+3y+5z=5x+6y+4z......x+z=3y\)
Subtract (3) from (2), \(2y-z=2z-3y......5y=3z\)
Multiply (1) by 2 and subtract (2) from it \(4x-x-2y=2y+2z-2z....3x-2y=2y.....3x=4y\)
Now we can find the ratio of all three sides. \(3x=4y .....y=\frac{3x}{4}\) and \(5y=3z.....z=\frac{5y}{3}=\frac{5}{3}*\frac{3x}{4}=\frac{5x}{4}\)
So \(x:y:z=x:\frac{3x}{4}:\frac{5x}{4}=4x:3x:5x=4:3:5\), which is
a triplet 3:4:5 in a right angled triangle. We are looking for angle ABC, which is opposite to AC or z.
As z is the largest side, angle ABC will be the largest or 90, as 90 is the largest angle in a right angled triangle. D