This question can't be answered. You didn't mention the amount of sand in the mixture. Until we know the exact amount of sand in the mixture, we can't answer it. Post this question again by mentioning the amount of sand. Lets suppose your question is like this. A mixture contain 1.50 g NH4Cl, 0.80 g NaCl, and 1.20 g SiO2. What is the percentage of sand in this mixture? In this case, you will do the calculation like this

Amount of sand / (total amount) x 100

1.2/ (1.2+0.8+1.5) X 100 = 34.28 %

Hope you understand the method.

Amount of sand / (total amount) x 100

1.2/ (1.2+0.8+1.5) X 100 = 34.28 %

Hope you understand the method.