Final answer:
Soo-Jung used the subtraction property of equality to isolate the variable y in an equation by subtracting the same amount from both sides. An example equation could be y + a = b, which, after subtracting a from both sides, becomes y = b - a.
Step-by-step explanation:
The student's question relates to using the subtraction property of equality to solve for a variable, in this case, y. The subtraction property of equality states that if two expressions are equal and you subtract the same amount from both sides, they remain equal. An example of an equation that Soo-Jung might have solved using this property is:
y + a = b
To solve for y, Soo-Jung would subtract a from both sides, resulting in:
y = b - a
This is a straightforward application of the subtraction property of equality to isolate the variable y. Another instance, incorporating the given phrases, could be to start with the equation:
y + 0.2y = $220
And then apply the property:
y = ($220 - 0.2y) / 0.8