To find the amount Mrs. Li was billed for her long-distance calls totaling 223 minutes, you can use the expression:
28 + 0.07 * 223
First, calculate the cost of the long-distance calls:
0.07 * 223 = 15.61
Now, add this cost to the fixed monthly charge:
28 + 15.61 = 43.61
So, Mrs. Li was billed $43.61 for her phone service with 223 minutes of long-distance calls.