You would leave a 0.5% tip
Step by step:
Since each smoothie cost $3 and you and your friend bought two smoothies, the total would come out to $6.
Since you and you’re friend split the total check of $7.20 (3.60 x 2) and the smoothies came out to be $6, subtract 6 from 7.20 and you would get 1.20.
You would then divide 1.20 by 2 (because the check was split in half) and that would give you 0.60 which is 50% of 1.20. You could also divide 60/120 to find the percentage.
Hope this helped :)