If we take the group of 25 boys then 26 girls will be acquainted with them. If we take a group of 24 boys then 1 girl from the group of 26 will not be chosen so she is acquainted only with the boy we left out from the group; we can chose the 24-group of boys in 25 ways by letting each time a boy out so 25 girls are acquainted each with only a boy. So, 1 girl can be acquainted with more then 2 boys. Now suppose that this girl is acquainted with at most 15 boys; if we chose a group of 10 boys by letting out 15 boys that the girl may be acquainted with, then the number of girls is 10 because each girl left is acquainted with only a boy, contradiction(the number of girls should be 11). So, this girl is acquainted with at least 16 boys.