If a:b=1:3,b:c=5:7 ,then what is the value of a:b:c?

A 3:5:7

B 5:9:15

C 5:15:21

D 4:13:24

Correct Answer: Option C

এখানে ,
a:b=1:3
      =(1×5):(3×5)
      =5:15

এবং b:c=5:7
             =(5×3):(7×3)
             =15:21
a:b:c=5:15:21