Answer :
[tex]3\ buckets\ to\ fill\ \frac{1}{3}\ of\ tank\\\\
x\ buckets\ to\ fill\ 1\ tank\\\\
proportion:\\
3-\frac{1}{3}\\
x-1\\\\cross\ multiplication:\\
3*1=\frac{1}{3}x\ \ \ | multiply\ by\ 3\\\\
x=9\\\\
To\ fill\ 1\ tank\ we\ need\ 9\ buckets.[/tex]