[The following information applies to the questions displayed below.] Grady received $8,200 of Social Security benefits this year. Grady also reported salary and interest income this year. What amount of the benefits must Grady include in his gross income under the following five independent situations? (Leave no answer blank. Enter zero if applicable.)

b. Grady files single and reports salary of $22,000 and interest income of $600

The formula given in my book is: 50%(MAGI + 50% of SS - $25,000)

Which for this problem would be .05($26,700 + 4,100) - 25,000 = $2,900

However the correct answer is calculated as (26,700 - 25,000) = $875.

Why is this?