The last equation describes this statement because if he earns 12.50 per hour the y will be the amount he earns in x hours. For every hour, the money will go up by $12.50.
The answer would be y=12.5x. If they were to ask a question that included how many hours he worked and you were trying to figure out how much he made in those hours you would plug the number of hours into x and solve it.
