Question

read this word problem.

kayla began a science experiment with 18 grams of an unknown solid. during the experiment, the change in the mass of the solid was -7 grams. what was the mass of the solid at the end of the experiment?

to solve the problem, edna subtracted 18 - (-7) and came up with an answer of 25 grams. is edna correct? why or why not?

yes, edna is correct.

no. the answer should have been -25 grams, not 25 grams, because the mass of the solid decreased.

no. edna should have added 18 + (-7) because she needed to add the initial mass and the change in mass.

Explanation:

Step1: Understand the problem

The initial mass is 18 grams, and the change in mass is -7 grams (a decrease of 7 grams). To find the final mass, we need to add the initial mass and the change in mass. So the formula should be \( \text{Final Mass} = \text{Initial Mass} + \text{Change in Mass} \), which is \( 18 + (-7) \).

Step2: Analyze Edna's solution

Edna used \( 18 - (-7) \), which is incorrect. Subtracting a negative is like adding a positive, but that would imply the mass increased, which contradicts the change being -7 (a decrease). The correct operation is adding the initial mass and the change (since change is -7, it's a subtraction in value but addition in the formula for final mass: \( 18 + (-7) = 11 \) grams).

Step3: Evaluate the options

• The first option is wrong because Edna's method is incorrect.
• The second option is wrong because the mass can't be -25 (mass is positive, and the change is -7 from 18, so it should be positive but less than 18).
• The third option is correct because to find the final mass, we add the initial mass (18) and the change in mass (-7), so \( 18 + (-7) \) is the correct operation.

Answer:

No. Edna should have added \( 18 + (-7) \) because she needed to add the initial mass and the change in mass.