Don't look at the numbers 29, 31, 30 for now. Just think about odds and evens.
If there are the same number of boys as girls, then is the total number of students an odd number or an even number? Is this always the case?
Now let's add one extra boy to the total. Is the new total odd or even?
Now look at your choices: 29, 31, 30. Which can you rule out?
Perhaps there is more than one right answer!
Stephen La Rocque.>