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: Analizar la operación correcta

Para encontrar la masa final, se debe sumar la masa inicial y el cambio en masa. La masa inicial es \(18\) gramos y el cambio es \(- 7\) gramos. La operación correcta es \(18+(-7)\).

Step2: Calcular \(18+(-7)\)

Según la regla de la adición de números enteros \(a+(-b)=a - b\) cuando \(a>b\). Aquí \(a = 18\) y \(b = 7\), entonces \(18+(-7)=18 - 7=11\) gramos.

Step3: Analizar el error de Edna

Edna hizo \(18-(-7)\). Según la regla \(a-(-b)=a + b\), entonces \(18-(-7)=18 + 7 = 25\) gramos. Pero el problema pide sumar la masa inicial y el cambio en masa (que es una adición, no una sustracción).

Answer:

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