Answer:
The "certain number" is 2.
Explanation:
Let x be the "certain number." The original fraction is (9/16). The certain number, x, is subtracted from the numerator (9-x) and added to the denominator (19+x) to yield a new fraction:
(9-x)/(19+x)
This is equal to (1/3)
(9-x)/(19+x) = 1/3
3(9-x)/(19+x) = 1
3(9-x) = (19+x)
27-3x = 19+x
4x = 8
x = 2