1) 17, with a difference of 4; 2)8 1, and the last number is three times that of the previous one; 4) 1 1,+1+2+3+4+5+6, add 1 in turn. If boys stand in a row, there are 4 girls out of 5 boys, 9 girls out of 10 boys, 19 girls out of 20 boys, 49 girls out of 50 boys and 99 girls out of 100 boys. Look at the interval, five people have four intervals, and the interval can be subtracted from the total 1. 68 divided by 4, minus 1 equals 1 1, just like the above question.