Answer: 285.63g of MgCl2.
Step-by-step explanation:
Very easy stiochemistry question. Use the dimensional analysis. For example 1 m x 100 cm / 1m and meters get canceled out and 1 m is 100 cm.
For the question, start with the given things. You know that it was started with 72.8 grams of magnesium. Convert it to molar mass (to use moles for comparison), and then find the mass of mg.