Answer : The coffee diluted was, 1 in 10, or 1/10, or 0.1 dilution factor.
Explanation :
Dilution : It is defined as the solute concentration of a solution decreased by adding the solvent.
The dilution factor for 1 ml of coffee into 9 ml of water is:

Now put all the given values in this expression, we get the dilution factor for coffee.

Therefore, the coffee diluted was, 1 in 10, or 1/10, or 0.1 dilution factor.