To calculate this, we can use complements. We can find, instead the probability that at least one will donate, the probability that not none of them will donate.
First, we can find the probability that none will donate, which will be (70%)^4, since the chance each will not donate is the same. From this we get 24.01%.
To find the probability that at least one donates, we do the complement:
The tax added to $220 is solved by 220 X .0725 = 15.95. Add 15.95 to $220 and get $235.95. The tip (assuming it is 20% after tax ) is $235.95 X .20= $47.19 Add the $47.19 to $235.95= $283.14
