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Let ABCA B C be a right-angled triangle with BCB C as the hypotenuse. Lengths of ABA B and AC are 15 km15 \mathrm{~km} and 20krn20 \mathrm{krn}, respectively. The minimum possible time, in minutes, required to reach the hypotenuse from A at a speed of 30 km30 \mathrm{~km} per hour is

Entered answer:

Solution

✅ Correct Answer: 24

We need to find the shortest path from point A to the hypotenuse BC.


Since ABC is a right-angled triangle with AB = 15 km and AC = 20 km, we can use the Pythagorean theorem to find BC:

BC² = AB² + AC²

BC² = 15² + 20²

= 225 + 400

= 625

BC = 25 km

Notice that 15, 20, 25 is just the famous 3-4-5 right triangle scaled up by 5!


The question asks for the minimum time to reach the hypotenuse. This means we need the shortest distance from A to line BC.

The shortest distance from any point to a line is always the perpendicular distance (the altitude). If you draw any other line from A to BC, it will be longer than the perpendicular.


For any right triangle, we can find the altitude to the hypotenuse using the area formula.

Area of triangle = 12\dfrac{1}{2} × base × height

We can calculate the area in two ways:

Using the two legs: Area = 12\dfrac{1}{2} × AB × AC = 12\dfrac{1}{2} × 15 × 20 = 150 km²

Using hypotenuse and altitude: Area = 12\dfrac{1}{2} × BC × altitude

Since both give the same area:

150 = 12\dfrac{1}{2} × 25 × altitude

altitude = 300 ÷ 25 = 12 km

In a 3-4-5 triangle, the altitude to hypotenuse = 3×45\dfrac{3×4}{5} = 2.4. In our 15-20-25 triangle (scaled by 5), it's 2.4 × 5 = 12 km


Now we use the basic formula: Time = Distance ÷ Speed

Time = 12 km ÷ 30 km/hr = 1230\dfrac{12}{30} hours = 25\dfrac{2}{5} hours

Converting to minutes: 25\dfrac{2}{5} × 60 = 24 minutes

Therefore, the minimum possible time is 24 minutes.


When finding the shortest path from a point to a line, always think "perpendicular distance" - it's a concept that appears frequently in geometry problems!

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