It is 28 pounds. This is because all of the others have a ratio of 4:1 while 28 and 5.5 have a ratio of 56:13.
Since there are two halves in a whole, you can times each number by 2 to get the number of halves it has.
12*2=24
10*2=20
13*2=26
15*2=30
8*2=16
5*2=10
He will eventually make enough money to buy his car
2/5ths divides by 3 and 1/10 is 0.129
