Noticed a typing error as soon as I hit send.  Corrected answer appears below:

While anikam’s answer is mathematically correct, he failed to take into account that after one and a half burgers, the boys were full and ate no more in the next one and a half hours.  Thus, the answer for nine boys is nine.  😉

While anikam’s answer is mathematically correct, he failed to take into account that after one and a half burgers, the boys were full and ate no more in the next one and a half hours.  Thus, the answer for nine boys is six.