${10}^{-4} g$ of gelatin is required to be added to $10{ cm}^3$ of a standard gold solution to just prevent precipitation by addition of $1{ cm}^3$ of $10\%NaCl$ solution to it. Hence, the gold number of gelatin is (decimal)

As we know that, gold number is the number of milligrams of dry protective colloids required to just prevent the coagulation of 10 mL of red gold sol when 1 mL of 10% NaCl added to it.

Amount of gelatin required to protect 10 mL gold solution is:

$\frac{{10}^{-4}\times 10}{10}=0.1 \mathrm{mg}$

Hence the gold number is $0.1$.