Final answer:
The error in the student's calculation was not correctly dividing by a negative fraction and then adding the result to -2.5. The correct answer should be -5.5.
Step-by-step explanation:
The student made an error in calculating -2.5 + 1 1/8 divided by (-3/4). To find the error, we first need to convert the mixed number to an improper fraction and then perform the division carefully, making sure to take into account the signs involved. The calculation should be as follows:
Convert 1 1/8 to an improper fraction: 1 1/8 = 9/8.
Divide 9/8 by -3/4, which is the same as multiplying 9/8 by the reciprocal of -3/4, which is -4/3.
Multiply 9/8 by -4/3: (9/8) * (-4/3) = -3/1 or -3.
Add this result to -2.5: -2.5 + (-3) = -5.5.
Therefore, the correct answer is -5.5, not -3 1/6 as originally stated by the student.