Can someone explain this and what the answer is?

How can you decide witch operations to use to slice a two step equation

Which shows the correct solution of the equation 1/2a+2/3b=50, when b=30?

Sandy is ordering bread rolls for her party,she wants 3/5 of the rolls to be whole wheat. what other factions can represent part of the rolls that will be whole wheat? shade the models to show your work.