You make use of the given relationships to write an equation only involving "a".
... a = (sum of three numbers) × (1 + 0.25) . . . . a is 25% more than the sum of three numbers.
Then
... (sum of three numbers) = a/1.25 = 0.8a
The sum of the three numbers and a is given as 783.
... sum of 4 numbers = (sum of three numbers) + a
... 783 = 0.8a + a = 1.8a
... 783/1.8 = a = 435
The appropriate selection is (B) 435.
