What is the next four terms in the arithmetic sequence: 12, 16, 20,...?

${a}_{4} = 24$
We know that in an arithmetic progression we have ${a}_{1} = 12 , {a}_{2} = 16 , {a}_{3} = 20$ and we're looking for ${a}_{4}$. In a arithetic sequence the common difference is given by $$d=a_i-a_{i-1}$$ for 'any working $i$.'
So: $$d=a_3-a_2=20-16=4$$ or as well $$d=a_2-a_1=16-12=4$$
Now simply $$a_4=a_3+d=20+4=24$$