a : b = 2 : 3
c : d = 5 : 6
Multiplying a :Â b by 5 and c : d by 3
=> a :Â b = 10 : 15 and c : d = 15 :Â 18
a :Â b : c = 10 : 15 : 18
c : d = 8 : 9
Multiplying a :Â b : c by 4 and c :Â d by 9
=> a : b : c = 40 : 60 : 72 and c : d = 72 : 81
Then, a : b : c : d = 40 : 60 : 72 : 81
Therefore, a : d = 40 : 81
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