Balance a model water store

Supplied materials

• All measurements and observations below are supplied classroom examples. They are not data from actual learners or a claim about a particular real site.
• Paper or browser document; an ordinary calculator is optional.
• Rule: Final store equals initial liters plus input liters minus output liters. Formula: a+b-c. Variable key: a = initial water (L), b = input water (L), c = output water (L)
• Distinct worked inputs and result: A distinct store begins with 90 L, gains 15 L and loses 20 L: 90+15-20=85 L.
• Assessment case 1: Supplied case 1: initial water (L)=100; input water (L)=20; output water (L)=30, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?
• Assessment case 2: Supplied case 2: initial water (L)=150; input water (L)=25; output water (L)=40, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?
• Assessment case 3: Supplied case 3: initial water (L)=200; input water (L)=30; output water (L)=20, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?
• Assessment case 4: Supplied case 4: initial water (L)=80; input water (L)=15; output water (L)=25, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?
• Assessment case 5: Supplied case 5: initial water (L)=120; input water (L)=40; output water (L)=30, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?
• Assessment case 6: Supplied case 6: initial water (L)=180; input water (L)=20; output water (L)=50, using the explicitly named quantities. Final store equals initial liters plus input liters minus output liters. What result follows?

Learn the idea

An accounting interval tracks input and output. A water store can decrease even while receiving recharge if withdrawals are greater. These volumes are fictional, with no unmeasured flows in the model.

Final store equals initial liters plus input liters minus output liters. The formula in the supplied model is a+b-c. Variable key: a = initial water (L), b = input water (L), c = output water (L). Include both incoming and outgoing water. The closed accounting model omits unknown flows.

A distinct store begins with 90 L, gains 15 L and loses 20 L: 90+15-20=85 L. A decrease is not evidence that recharge is zero. Track the quantity and unit before calculating. Explain the meaning of a positive, negative or fractional result when relevant. A numerical answer supports the stated model calculation; broader claims require the additional measurements and assumptions described here.

1. Name the measured quantities and their units before substituting values.
2. Write the stated rule and use the variable key to match each symbol to its explicitly named quantity.
3. Calculate each supplied case separately, keeping intermediate arithmetic and the result unit.
4. Explain the result in context and identify the assumption that would invalidate direct use of the rule.

Check your reasoning

Your answer is temporary and is not saved to an account. Download your current answer before leaving or reloading. Written work on paper or in another app stays in your own file.

Take it further

Changed-input task: use the same stated model with initial water (L)=101; input water (L)=20; output water (L)=30. Expected result: 91 L. Explain what changes and keep this assumption: A decrease is not evidence that recharge is zero.

Nothing here is saved to an account or sent to a teacher. The answer download includes your current choice and any checked feedback. It does not include choices from the separate More practice section. Download that section separately.