X people answered at least two questions including the first question, and m people answered the third question without answering the first question. Then according to condition 2) 2m people answered the second question without answering the first question, according to condition 3) x+ 1 people only answered the first question, according to condition 4) x+ 1 people only answered the second question or the third question. 2x+2 people answered only one question. According to the condition 1), it is obvious that 3x+2 is less than or equal to 40 people. 3m+2x+ 1 40 people or more. And 3m is greater than or equal to x+ 1. Obviously, x and m are integers. Let's try to make 3m+x+ 1 even and (3m+x+1)/2+x+1+x = 40. The only qualified solution is the answer ~ ~ X = 10.