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Plane $A$ is descending toward the local airport at a rate of 2,500 feet/minute. It is currently at an altitude of 12,000 feet. Plane $B$ is ascending from the same airport at a rate of 4,000 feet/minute and is currently at an altitude of 1,000 feet.

In approximately how many minutes will the two planes be at the same altitude? At what altitude will they be?

Let $x$ represent the time in minutes and $y$ represent the altitude in thousands of feet.

Equations:
\[
\begin{align*}
y &= 12 - 2.5x \\
y &= 1 + 4x
\end{align*}
\]

To solve these equations, graph the lines of the two equations and mark the intersection of the two lines.

This point is located at approximately $(4.25, 6.5)$, which means both planes will be at an altitude of about 6,500 feet after approximately 4 minutes and 15 seconds.

Answer :

We begin with the two equations that give the altitude (in thousands of feet) of the two planes as functions of time (in minutes):

$$
\text{Plane A: } y = 12 - 2.5x
$$
$$
\text{Plane B: } y = 1 + 4x
$$

Since the planes will be at the same altitude when their $y$-values are equal, we set the equations equal to each other:

$$
12 - 2.5x = 1 + 4x
$$

**Step 1. Solve for $x$**

First, subtract $1$ from $12$:
$$
12 - 1 = 11
$$

Now, rewrite the equation by collecting the $x$ terms on one side:
$$
11 = 4x + 2.5x
$$

Combine like terms:
$$
11 = 6.5x
$$

Solve for $x$ by dividing both sides by $6.5$:
$$
x = \frac{11}{6.5} \approx 1.6923 \text{ minutes}
$$

**Step 2. Find the altitude where they meet**

It is convenient to use the equation for Plane B. Substitute $x \approx 1.6923$ into the equation for Plane B:

$$
y = 1 + 4x \approx 1 + 4(1.6923) \approx 1 + 6.7692 \approx 7.7692
$$

Since $y$ is given in thousands of feet, the altitude in feet is:

$$
\text{Altitude} \approx 7.7692 \times 1000 \approx 7769 \text{ feet}
$$

**Step 3. Convert the time into minutes and seconds**

We have $x \approx 1.6923$ minutes. The whole number part represents the minutes. So the time is 1 minute plus a fractional part:

- Minutes: $1$
- Fractional Part: $0.6923$ minutes

To convert the fractional part into seconds, multiply by 60:

$$
\text{Seconds} = 0.6923 \times 60 \approx 41.54 \text{ seconds}
$$

**Final Answer**

The two planes will be at the same altitude after approximately $1.69$ minutes (or $1$ minute and $41.5$ seconds). At that time, they will both be at an altitude of about $7769$ feet.

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Rewritten by : Barada