If the auditorium is as described, then the first row has $18$ seats, the second row has $20$ seats, the third row has $22$, the fourth row has $24$, and so on. You can see that the $21$-th row has $18+(2)(20)$ seats, that is, $58$ seats. Now you can certainly add up the even numbers from $18$ to $58$. It takes a while.
But the person who described the auditorium lied. Actually, it has the same number of seats in every row, $18+58$, and it is full. In the first row, the first $18$ people are boys, the remaining $58$ are girls. In the second row, the first $20$ are boys, the rest are girls. In the third row, the first $22$ are boys, the rest are girls. This pattern goes on until the very back, where the first $58$ are boys and the remaining $18$ are girls. Perhaps you can make a sketch of the auditorium, and the seating pattern.
Now can you tell me quickly how many boys there are?