The ratio of three number is 3 : 4 : 5 and their LCM is 2400, their HCF is:

40 80 120 200

40

3   :    4   :    5    ⇒   60 (L.C.M.)                                  ↓ × 40 = 1R                                2400   1R= H.C.F. = 40