Butter fingers

This task is about explaining calculation errors.
Doug was using a calculator to do his maths but he made lots of mistakes. He always got the first number right but got the operation or the second number wrong. In each case explain the mistake he made. The first one has been done for you.
 
Question:
Doug's answer:
Mistake:
7,123 + 8,641
14 764

Question 1Change answer

a) Question: 89 – ⁻23
  Doug's answer: 66
 
Mistake:

Question 1Change answer

b) Question: 17 × 56
  Doug's answer: 0.303571428
 
Mistake:

Question 1Change answer

c) Question: 35 × 25%
  Doug's answer: 875
 
Mistake:

Question 1Change answer

d) Question: 7.25 + 5.602
  Doug's answer: 12.87
 
 
Mistake: