Well, assuming Joy is Gias' sister, first we subtract 4 from the whole: 74 - 4 = 70. Then, we devide by 2. 70/2 =35. Then, for Joy's age, we add 4: 35 + 4 = 39. So Gias is 35 and Joy is 39. 35 + 39 = 74.
Otherwise, you never said Joy is Gias' sister. So in the case that it is a trick question, Joy could be anything... :-)