The average of six numbers is 10
and the numbers are 5, 8, 12, 15, 17
so when this numbers are added and divided with 6 we should get 10
which means, 5+8+12+15+17+x =10
6
where x is the sixth number
so 57+x =10
6
57+3= 60/6 =10
6
so the sixth number is 3.