Going out on a limb here and guessing that the function is

Please correct me if this isn't the case.
Recall that

which converges for

.
It follows that

Do a proportion
65/100= x/80
Cross multiply and divide
80 x 65= 5, 200
5, 200 divided by 100 is 52
52 members left
80-52= 28
Answer is 28 members
You left 20% tip. all you do is divide the tip,0.60, by the total , 3.
ik the first answer choice is false