You would need to know how much it cost to rent each type of tube in order to solve this problem.
One of the equations would be xx+yy=15
The number of single tubes plus the number of cooler tubes is 15.
If you know how much each tube cost, simply put the cost in front of the xx/yy and set it equal to 270$. For example, if the single tubes cost 15$ and the cooler tubes cost 20$, then the equation would be 15xx+20yy=270