If the 6^th term of H.P is 10, and the 11^th term is 18. Then the 16^th term is 

Here $a+5d=\frac{1}{10}$ and $a+10d=\frac{1}{18}$

Solving these two equations, we get $a=\frac{13}{90}$ and  $d=\frac{-2}{225}$

We have $a+15d=\frac{13}{90}-\frac{2}{15}=\frac{1}{90}$

Hence the 16^th term is 90