A fruit drink contains 90.% of water and 10.% fruit juice. How much fruit juice does 500.0ml of fruit drink contain ?

90% is 9 parts out of every 10, and similarly, 10% is 1 part out of every 10. Split 500 mL into 10 equal parts and you get 50mL each part. One of those parts is equal to 10%, meaning your final answer is 50. mL accounting for significant figures.