What mass will an atom with 30 protons 30 electrons and 29 neutrons have?

Iknow the answer is a but i don't know why

Ammonium perchlorate nh4clo4 is the solid rocket fuel used by the u.s. space shuttle. it reacts with itself to produce nitrogen gas n2 , chlorine gas cl2 , oxygen gas o2 , water h2o , and a great deal of energy. what mass of nitrogen gas is produced by the reaction of 2.1g of ammonium perchlorate?

What causes most sediment to wash or fall into a river