What is the mass number of an atom which contains 21 electrons,21 protons and 24 neutrons

When the water vapor cools it condenses select a number that represents his process on the

Silver tarnishes as silver metal reacts with hydrogen sulfide, h2s, in the air. in this reaction, dark silver sulfide, au2s, covers the surface of silver. when silver is polished, this coating of silver sulfide can be removed from the surface. this makes the silver shiny again. enter the coefficients that balance the tarnishing reaction equation. (type 1 for no coefficient.)

How many grams of mgbr2 are needed to produce 75g or metal?