數學maths一問

3a = 5b = 7c
(3a/ 3*5*7) = (5b/ 3*5*7) = (7c/ 3*5*7)
a/ 35 = b/ 21 = c/15

a/ 35 = b/21................and..............b/21 = c/15
21a = 35b.....................................15b = 21c
a:b = 35:21....................................b:c = 21:15

a:b = 35:21
...b:c = ...21:15
a:b:c = 35 : 21 : 15

ANSWER : D

above is a silly method,
your calculation can be stopped
when you can find out a/ 35 = b/ 21 = c/15
because you can see that a=35k and b=21k and c=15k ,where k is a non-zero constant
then a:b:c = 35k : 21k : 15k = 35:21:15

5*7:3*7:3*5
=35:21:15
D!