Answer:
y ∈ ℝ
Explanation:
You want the solution to the equation 0.4(3y + 18) = 1.2y + 7.2.
Simplify
The parentheses can be removed by making use of the distributive property.
0.4(3y + 18) = 1.2y + 7.2 . . . . . . given
0.4(3y) +0.4(18) = 1.2y +7.2
1.2y +7.2 = 1.2y +7.2 . . . . . . . . . true for any value of y
The set of solutions for y is all real numbers.
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Additional comment
Actually, the solution set is "all complex numbers" as well as any other entities for which multiplication and addition with scalars are defined. For example, y could be a matrix of complex numbers, and the equation would still be true.