1/12 + 1/6x = 3/4...multiply everything by common denominator of 12
1 + 2x = 9
2x = 9 - 1
2x = 8
x = 8/2
x = 4 <== he can play 4 rounds

23 stones will be needed (you have to round up, since you can't have a fraction of a stone)
It’s 152y+125, basically by simplifying
2 bags, because when you add 5/8 and 1 3/8 you get 1 8/8 which is really 2
12/3 = 4
4 cars per hour
4 x 5 = $20
20 x 8 = $160 for 8 hrs