If s is the price of a streamer roll and b the price of a balloon, 3s+15b=30, so s+5b=10.
There’s insufficient information to find the cost of each item, but we can relate the cost of each, s=10-5b=5(2-b) or b=(10-s)/5.
For example, if a balloon costs 50c, then a streamer roll costs 5(2-0.50)=5×1.5=$7.50.
If a balloon costs $1, a streamer roll costs $5. If a balloon costs $1.50, a streamer roll costs $2.50.
A balloon must cost less than $2 and a streamer must cost less than $10.