so 30/48=15/(x+9)
this could be easily solved by recognizing the proportion (the other way is usiing full algebra and doing multiply both sides by 48(x+9) and then divide both sides by 30 then subtract 9)
30 is 2 times of 15
therefor 48 is 2 times of (x+9)
48=2 times (x+9)
divide both sides by 2
24=x+9
subtract 9
15=x
x=15