“weight of the dried protective agent in milligrams, which when added to 10 ml of a standard gold sol (0.0053 to 0.0058%) is just sufficient to prevent a colour change from red to blue on the addition of 1 ml of 10 % sodium chloride solution, is equal to the gold number of that protective colloid.”

0.25 g lyophilic colloid is added to 100 ml gold solution

so 250 mg lyophilic colloid is added to 100 ml gold solution

so 25 mg lyophilic colloid is added to 10 ml gold solution

so answer will be 25