Right Triangle Positions of Moving Ships

CAT 2022 Slot 3 · Quantitative Ability · Hard · Geometry

Two ships are approaching a port along straight routes at constant speeds. Initially, the two ships and the port formed an equilateral triangle with sides of length 24 km. When the slower ship travelled 8 km, the triangle formed by the new positions of the two ships and the port became right-angled. When the faster ship reaches the port, the distance, in km, between the other ship and the port will be

  1. A.

    8

  2. B.

    12

  3. C.

    6

  4. D.

    4

Answer

B

Explanation

Let the port be PP, slower ship be S1S_1, faster ship be S2S_2. Initially, PS1=PS2=24 kmPS_1 = PS_2 = 24\text{ km}, angle ∠S1PS2=60∘\angle S_1 P S_2 = 60^\circ.

When S1S_1 travels 8 km towards PP, its distance from PP becomes 24−8=16 km24 - 8 = 16\text{ km}. Let xx be the distance traveled by S2S_2. Its distance from PP becomes 24−x24 - x.

In △S1′PS2′\triangle S_1' P S_2', sides are 1616 and 24−x24 - x with included angle 60∘60^\circ. For this triangle to be right-angled, either angle at S1′S_1' or at S2′S_2' is 90∘90^\circ.

Case 1: Angle at S1′S_1' is 90∘90^\circ. In right △S1′PS2′\triangle S_1' P S_2' with ∠P=60∘\angle P = 60^\circ: cos⁡60∘=PS1′PS2′=1624−x=12  ⟹  24−x=32  ⟹  x=−8 (invalid)\cos 60^\circ = \frac{P S_1'}{P S_2'} = \frac{16}{24 - x} = \frac{1}{2} \implies 24 - x = 32 \implies x = -8 \text{ (invalid)}

Case 2: Angle at S2′S_2' is 90∘90^\circ. cos⁡60∘=PS2′PS1′=24−x16=12  ⟹  24−x=8  ⟹  x=16\cos 60^\circ = \frac{P S_2'}{P S_1'} = \frac{24 - x}{16} = \frac{1}{2} \implies 24 - x = 8 \implies x = 16

So S2S_2 traveled 16 km while S1S_1 traveled 8 km. Speed ratio v2/v1=16/8=2v_2 / v_1 = 16 / 8 = 2.

When the faster ship S2S_2 reaches the port (travels full 24 km), the slower ship S1S_1 travels 24/2=12 km24 / 2 = 12\text{ km}. Distance between S1S_1 and the port =24−12=12 km= 24 - 12 = 12\text{ km}.

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