Answer:
In order to solve for this you would need to plug it into an equation. You would add them all up to eachother after multiplying them by their percentages.
So it would be (0.75*48.90)+(0.18*49.10)+(0.07*50.20). Plug it all into a calculator and you get 49.027amu. Then you would round to significant figures so 49.03amu should be your answer.