Answer:
given, 
 ratio= 2:3:5 , smallest part length=6
let x be the common multiple then,
first number (smallest) =2x
 6cm =2x 
 6/2=x
 therefore, x=3
now, 
1st part=2x=2*3=6
2nd part=3x=3*3=9
3rd part=5x=5*3=15
again, 
total length = sum of all part
 =6+9+15
 =30 
therefore 30cm is the answer