A ratio between two numbers is 3 : 4 and their L.C.M. is 84. The first number is  Pick one.
A) 60
B) 45
C) 21
D) 15
E) none

Rationale: "sol . let the required numbers be 3 x and 4 x . then , their l . c . m . is 12 x . ∴ 12 x = 84 ⇔ x = 7 . hence , the first number is 21 . answer c"
Answer: C