in an alpha decay, the atomic number reduces by 2 while the mass number decrases by 4, even though no example is given, we can assume , the example below to show you how to solve an alpha decay equation,
Therefore anytime you see an alpha decay decay just subtract four from the mass number and 2 from the atomic number, find an atom that fall within this , find an element in the periodic table with atomic number of less tan 2 of the parent atom .
http://sasuwaphysics..com.ng/2016/10/which-missing-item-would-complete-this_24.html