When a particular number is subtracted from each of 7, 9,

When a particular number is subtracted from each of 7, 9, 11 and 15, the resulting numbers are in proportion. The number to be subtracted is :

Let the number to be subtracted be x.
According to the question,

7 − x	=	11 − x
9 − x	=	15 − x

Now, check through options
Clearly, putting x = 3,

Each ratio =	2	.
	3

Note : Solve such questions orally by mental exercise.
Second Method :
The number will be x

=	ad − bc
	(a + d) − (a − d)

=	7 × 15 − 9 × 11
	(7 + 15) − (9 + 11)

=	105 − 99
	22 − 20

=	6	= 3
	2