Brittany, Alex, and Jamy all share 600 marbles divided between them in the ratio 3:5:7. If Brittany gives Alex half of her marbles, what's the total number of marbles that Alex has?
Work out the solution step by step. 
The total ratio representing the number of marbles is 3+5 +7 = 15
From the ratio, the fraction representing the number of marbles that Brittany has is 3/15, which is equal to 3/15*600 = 120 marbles.
Alex has 5/15*600 = 200 marbles.
If Brittany gives half of her marbles to Alex, Alex receives 1/2*120 = 60 marbles.
After receiving 60 marbles from Brittany, Alex has 200+60 = 260 marbles.