Answer: Choice B
They did not perform the cross multiplication properly
The correct step should be m*24 = 8*15 which solves to m = 5
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Explanation:
In general, if we had the equation A/B = C/D, then it cross multiplies to A*D = B*C. We refer to this as cross multiplication because of the shape made when we connect the terms (to figure out what multiplies with what). The diagram below shows what I mean.
So that's why m/8 = 15/24 cross multiplies to m*24 = 8*15
Simplifying things a bit leads to 24m = 120. Then we divide both sides by 24 to fully isolate m and we get m = 120/24 = 5. Therefore, m = 5.