Math Problem Statement

A train leaves station A at 5:00 am and reaches station B at 9:00 am on the same day. Another train leaves station B at 7:00 am and reaches station A at 10:30 am on the same day. The time at which the two trains cross one another is

A. 8:00 am

B. 8:26 am

C. 7:26 am

D. 7:56 am

Solution

To solve this problem, we first need to calculate the speeds of the two trains and then determine the time they meet. Let the distance between Station A and Station B be DD (in kilometers). Here’s the step-by-step process:

Step 1: Calculate the speed of the first train

The first train travels from A to B in 4 hours (from 5:00 am to 9:00 am). Its speed is: Speed1=D4km/h\text{Speed}_1 = \frac{D}{4} \, \text{km/h}

Step 2: Calculate the speed of the second train

The second train travels from B to A in 3.53.5 hours (from 7:00 am to 10:30 am). Its speed is: Speed2=D3.5=2D7km/h\text{Speed}_2 = \frac{D}{3.5} = \frac{2D}{7} \, \text{km/h}

Step 3: Write the relative positions as functions of time

Let tt represent the time (in hours) after 7:00 am when the two trains meet. At this moment:

  1. The first train has already been traveling for t+2t + 2 hours (it started at 5:00 am).
  2. The second train has been traveling for tt hours (it started at 7:00 am).

The distance covered by the first train is: Distance1=Speed1×(t+2)=D4×(t+2)\text{Distance}_1 = \text{Speed}_1 \times (t + 2) = \frac{D}{4} \times (t + 2)

The distance covered by the second train is: Distance2=Speed2×t=2D7×t\text{Distance}_2 = \text{Speed}_2 \times t = \frac{2D}{7} \times t

Since the two trains meet, the sum of these distances equals DD: Distance1+Distance2=D\text{Distance}_1 + \text{Distance}_2 = D

Step 4: Solve the equation

Substitute the expressions for Distance1\text{Distance}_1 and Distance2\text{Distance}_2: D4×(t+2)+2D7×t=D\frac{D}{4} \times (t + 2) + \frac{2D}{7} \times t = D

Divide through by DD (since D0D \neq 0): t+24+2t7=1\frac{t + 2}{4} + \frac{2t}{7} = 1

Eliminate the fractions by multiplying through by 28 (the least common multiple of 4 and 7): 7(t+2)+8(2t)=287(t + 2) + 8(2t) = 28

Simplify: 7t+14+16t=287t + 14 + 16t = 28

Combine terms: 23t+14=2823t + 14 = 28

Solve for tt: 23t=14t=1423hours23t = 14 \quad \Rightarrow \quad t = \frac{14}{23} \, \text{hours}

Step 5: Convert tt to minutes

t=1423×6036.52minutest = \frac{14}{23} \times 60 \approx 36.52 \, \text{minutes}

Thus, the two trains meet approximately 36.52minutes36.52 \, \text{minutes} after 7:00 am. This is around 7:37 am.

Correct Answer:

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Math Problem Analysis

Mathematical Concepts

Speed and Time
Relative Motion
Algebra

Formulas

Speed = Distance / Time
Relative Motion: Distance1 + Distance2 = Total Distance

Theorems

Relative Motion Theorem
Linear Equation Solving

Suitable Grade Level

Grades 9-11