If the numbers $$ \root 3 \of {9} , \root 4 \of 20 , \root 6 \of {25} $$   are arranged in ascending order, then the right arrangement is -

$$\root 3 \of 9 ,\,\,\root 4 \of {20} ,\,\,\root 6 \of {25} $$
Taking L.C.M of 3, 4 and 6
$$\eqalign{ & = \root {12} \of {{9^4}} ,\,\,\root {12} \of {{{20}^3}} ,\,\,\root {12} \of {{{25}^2}} \cr & = \root {12} \of {6561} ,\,\,\root {12} \of {8000} ,\,\,\root {12} \of {625} \cr} $$
So, ascending order
$$ = \root 6 \of {25} , < \root 3 \of 9 , < \root 4 \of {20} $$