There are at least 6 * 10 faucets leaking in the city, and one faucet leaks one cubic meter of water a month.

The water lost from the faucet is: a * 6 *10.5 cubic meters of water.

There are at least 2 * 10 5 toilets leaking in the city, and one toilet leaks 3a/2 cubic meters a month.

The water loss of toilet is (3a/2) * 2 *10.5.

The sum of the two is the minimum water loss of the city in one month.

9 a *10.5 cubic meters