Aquarium Care: Variable Plans

Use variables to describe tank care and feeding schedules.

Define the Variable

Read each situation independently. Choose or match the variable model that fits.

An aquarium keeper records an unknown number of fish in a tank. Which definition lets n represent that unknown quantity?

A. n is the number of liters in the tank.B. n is the number of fish in the tank.C. n is the number of tanks in the room.D. n is the number of minutes until feeding.

Each tank receives 3 feeding checks per day. The aquarium may have 4, 5, or 6 tanks open to visitors. Let n represent the number of open tanks. Which statement is true?

A. n must equal 6 because 6 is the largest choice.B. n can equal 4, 5, or 6, depending on the day.C. n must equal 15 because 4 + 5 + 6 = 15.D. n must equal 3 because there are 3 checks.

Priya has measured x grams of fish food. She measures 7 more grams into the same container. Which expression represents the total grams of food?

A. 7 − xB. 7xC. x + 7D. x − 7

For each situation, x is the number of grams of food in a container at the start. Match the situation to an expression for the grams of food afterward.

A keeper may prepare feeding portions for 2, 4, or 6 tanks. Let n be the number of tanks. Each tank needs 5 grams of food. Select every expression that represents the total grams needed for any allowed value of n.

Model the Care Plan

Choose an expression for each quantity. Keep track of what happens first.

Omar starts with 48 grams of food and uses x grams during the morning feeding. Which expression represents the grams left?

A. x − 48B. 48 + xC. 48 − xD. 48 ÷ x

For a partial water change, a keeper prepares x liters of water and divides it equally among 6 tanks. Which expression represents the liters added to each tank?

A. 6 ÷ xB. 6xC. x − 6D. x ÷ 6

The table shows a feeding plan. Let n be the number of tanks. Which expression represents the total grams of food prepared?

Daily Food Preparation
Number of tanks	Total food (g)
2	15
4	27
7	45

The keeper prepares 6 grams per tank, plus 3 extra grams in a separate reserve cup.

A. 6n + 3B. 9nC. 6(n + 3)D. 3n + 6

Lena has n tanks to inspect. She finishes inspecting 2 tanks. Each remaining tank takes 4 minutes to inspect. Which expression represents the minutes needed to inspect all the remaining tanks?

A. 4(n − 2)B. 4n − 2C. 4(n + 2)D. n − 2 × 4

10.

A feeding period starts x minutes after noon and lasts 12 minutes. Which expression represents how many minutes after noon the feeding period ends?

A. x − 12B. x + 12C. 12xD. 12 − x

Check and Create Models

Use the meaning of each variable to check the model. Explain your reasoning in the final question.

11.

A keeper removes x liters of water from a tank that contains 70 liters, then adds 9 liters of clean water. Which expression represents the liters in the tank afterward?

A. 70x + 9B. 70 + x − 9C. 70 − (x + 9)D. 70 − x + 9

12.

The expression 2x + 5 represents a care plan. Select every situation that this expression models.

13.

A feeder starts with 90 grams of food. It releases 6 grams at each feeding. Let n be the number of completed feedings. Which expression represents the grams remaining?

A. 90 − 6nB. 6n − 90C. 90 − 6 − nD. (90 − 6)n

14.

A keeper has x grams of food. She sets aside 10 grams for a special tank, then divides the rest equally among 3 other tanks. Mateo writes x ÷ 3 − 10 for the grams each of those 3 tanks receives. Write a correct expression and explain why Mateo's expression does not match the plan.