a:b=2:3

b:c=4:5

then a:c=

a : b = 2 : 3  and  b : c = 4 : 5

Therefore  :  a / b = 2 / 3  and  b / c = 4 /5

Now  Cross multiply each :

  3a  = 2b  and  5b = 4c

Now :  b = (3/2)*a  and  b = (4/5)*c

Comparing both :  (3/2)*a = (4/5)*c

  a / c = (4/5)*(2/3)

 a / c = 8 / 15

Therefore :  a : c = 8 : 15 

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