Final answer:
Sunil's debt D after x days is represented by the equation D = $500 - $20 × x, which accounts for him paying off half of his daily earnings of $40 towards the $500 debt.
Step-by-step explanation:
The equation to represent Sunil's debt D after x days can be derived from knowing that he owes Cara $500 and pays half of his daily income, which is $40, to repay the debt. This means Sunil pays $20 per day towards the debt. Therefore, the debt after x days will be calculated by subtracting $20 times the number of days, x, from the initial debt.
The equation representing Sunil's debt D after x days is:
D = $500 - $20 × x