Suppose Moosa is ( x ) years old ,
Thus Moosa's sister is ( 3x - 6 ) years old .
The sum of their ages is 30 :
( x ) + ( 3x - 6 ) = 30 ==》 4x - 6 = 30
==》 4x = 36
==》 x = 9
So Moosa is 9 years old .
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