154877

19
first, add 15 and 5 together. do the same for 7, 9 and 17. you should get 20 and 33.
find the difference between 33 and 20. it is 13. thirteen is the mean for both.

then times 13 and 3 or 4 together. you should get 39 and 52 respectively.

finally, subtract 39 or 52 from 20 or 33 respectively. you will get 19.