If the bill is 5 shillings too much, the 3 men need to divide the 5 between themselves. Everyone gets 
back or rewritten
. As a customer you pay 10 and get back. Effectively you pay
.
The final bill per person is 
. Lets reverse. 
(even more simplified: 
and 
) original bill was 30 with the 5 reduction makes it 25.
Now the waiter pockets 2 shillings. He burdens 3 people with 2 shillings. Thus every man donates an extra 
for the waiter. Lets double check 
. Indeed waiter gets 2 extra.
The first statement is correct. Every man pays 9 shillings. and a bonus burden of making it an even 9 per person.
The second statement is incorrect. They did not pay 30. They paid 27. Officially 25, but an extra 2 donation to the waiter makes it a final bill of 27.
Great deduction!
When I figured out the solution, I was in the middle of driving.
I, too, realized the men didn't actually pay 27 shillings, they paid 25 shillings.
25/3 = 8 1/3
Then each had 1 shilling returned, so they paid effectively 9 1/3 shillings. That is 28, plus the 2 the waiter stole, =30