154684

Firstly, I worked out the mean of the first three numbers:1/3(15+5+x). This must be equal to the mean of the last four numbers: 1/4(x+7+9+17), giving the identity 1/3(15+5+x)=1/4(x+7+9+17). I worked out the value of x by multiplying both sides by the LCM which is 12, giving:80+4x=3x+99. From here, by subtracting 3x and 80 from both sides,x=19.