When a list of 5 numbers $a, b, c, d, e$ has the property that $a+b+c=c+d+e$, it is also true that $a+b=d+e$. With the given list of 5 numbers, it is likely easier to find two pairs with no overlap and with equal sum than to find two triples with one overlap and equal sum. After some trial and error, we can see that $6+21=10+17$, and so the list $6,21,5,10,17$ has the given property, which means that 5 is in the middle.