Solution:The pattern followed here is :1st number + 50 = 2nd number & 2nd number + 50 = 3rd number
(20, 70, 120) → 20 + 50 = 70 & 70 + 50 = 120
(120, 170, 220) → 120 + 50 = 170 & 170 + 50 = 220
Similarly,
(420, 470, 520) → 420 + 50 = 470 & 470 + 50 = 520
Hence, "(420, 470, 520)" is the correct answer.