Final answer:
Chris earns $77 in a week when he sells 26 books, as calculated by the equation A = 51 + b.
Step-by-step explanation:
The amount that Chris earns in a week is given by the equation A = 51 + b, where A represents total earnings and b represents the income from selling b books. To calculate Chris's earnings for selling 26 books, we simply substitute b with 26 in the equation.
So, A = 51 + 26
A = 77
Therefore, Chris earns $77 in a week if he sells 26 books.