Only half of the 16 boys are using the computers, so, 16/2 = 8
All of the 13 girls are using computers.
8 + 13 = 21
That leaves us with 21 people, but since these people will share computers, (in pairs) we divide 21 by 2.
21/2 = 10.5
You can’t have half of a computer, so you round up so there is still enough computers for everyone, therefore, 11 computers will be needed.