Answer: there are 36 tourists.
Step-by-step explanation:
Ok, we have two integer numbers:
T, the number of tourists and S, the number of seats.
Such that, if we divide the tourists into groups of 4 and in groups of 6, we end with no empty seats.
Knowing that the number S must be an integer, and when we divide T into groups of 4 and 6 we end with no empty seats, this means that T can be divided by 4 and by 6.
This means that T is a common multiple of 4 and 6
Now, we also know that 25 < T < 45.
Then let's see which ones are the multiples of 4 between 25 and 45.
4*7 = 28
4*8 = 32
4*9 = 36
4*10 = 40
4*11 = 44.
Now, the multiples of 6 in that range are:
6*5 = 30
6*6 = 36
6*7 = 42.
You can see that 36 is a common multiple, other way to see it is:
4*9 = (2*2)*(3*3) = 2*2*3*3 = (2*3)*(2*3) = 6*6
So the numer of tourist must be 36.
. Reminder: 
(usually used when finding out the area of a circle)
or 
or 
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