Mon wants to make 5 lbs of the sugar syrup. How much water and how much sugar does he need to make 50% syrup?

A 50% syrup solution would be half water and half sugar.  In other words, there'd be the same weight of sugar as there would be of water.  Let this weight be x.  Then 2x=5 lb, and x = 5/2 lb.

Use 5/2 lb of water and 5/2 lb of sugar to make this 50% syrup.