Задачі на прогресії/1

$$

a_8-a_5 = 27, a_1 = 2, a_{20} - ?

a_5 = a+4d, a_8 = a+7d, a_8 - a_5 = 3d = 27, d = 9

a_{20} = a+19d = 2+19 \cdot 9 = 2+90+81 = 173 $$