If Mary's income (after the increase) is 55% of the combined total, that means that John's income (which is fixed at 18,000), must be 45% of the total (T).

So $18,000=.45t$, or $t = \frac{18,000}{0.45} = 40,000$.

That means that Mary's new income would be $40,000 - 18,000 = 22,000$, which is $$7,000$ more than she has now.

Answer: C

Mary’s annual income is $15,000 and John’s annual income is $18,000. By how much must Mary’s annual income increase so that it constitutes 55% of Mary and John’s combined income?

(A) $3,000
(B) $4,000
(C) $7,000
(D) $11,000
(E) $25,000

Given that Mary’s annual income is $15,000 and John’s annual income is $18,000. And we need to find By how much must Mary’s annual income increase so that it constitutes 55% of Mary and John’s combined income?

Let Mary's income increase by x, then the equation will be
15,000 + x= (55/100)* (15,000 + x + 18000 )
15,000 + x = (11/20) * (33,000 + x)
300,000 + 20x = 11x + 363,000
9 x = 63,000
x = 7,000

So, Answer will be C
Hope it helps!