# Differential Equations

### Topics

There are three steps to solving a math problem.

- Figure out what the problem is asking.

- Solve the problem.

- Check the answer.

### Sample Problem

Model the following situation with a differential equation: A bucket starts out with 5 gallons of water. A diluted mixture of red dye (40 percent red dye; 60 percent water) is dripping into the bucket at a rate of 1.5 gallons per minute. The dye mixture swirls around and mixes evenly with the liquid already in the bucket, then drips out a hole in the bottom of the bucket at a rate of 1.5 gallons per minute.

Answer.

• Figure out what the problem is asking.

Let's figure out the general situation. We have a bucket.

Red dye is dripping into the bucket, mixing, then dripping out again.

We're interested in the quantity of red dye in the water. We're given numbers for how fast the liquid is flowing and how much red dye is in the incoming liquid, and we need to translate this into math.

• Solve the problem.

Let R stand for the amount of red dye in the water, measured in gallons, and let *t* stand for time (measured in minutes). We know that

Liquid is dripping into the bucket at a rate of 1.5 gallons per minute, but only 40 percent of this liquid is red dye. Every minute, the amount of red dye that enters the bucket is

0.4(1.5) = 0.6 gallons.

Liquid is dripping out of the bucket at a rate of 1.5 gallons per minute. We need to know what percent of this liquid is red dye. The amount of red dye in the bucket is given by R. Since there are 5 gallons of liquid in the bucket, the percentage of the liquid in the bucket that is red dye is

If 1.5 gallons of liquid flow out of the bucket every minute, and of those gallons are red dye, the rate at which red dye flows out of the bucket is

Putting this information together, we have

• Check the answer.

This particular question doesn't really have an answer to check. After you learn how to solve differential equations, you'll get asked questions like "how much red dye is in the bucket after five minutes" or something like that. We'll give you ways to check answers to those sorts of problems when we get there.