The classic mistake is failing to check your answer. Double-check your addition!
You could try a visual approach, using bars of equal length to start with both items at the same price, then adjusting to make the desired price difference.
You could even try dusting off your algebra skills:
bat + ball = 110
bat = ball + 100
The ball costs 5p and the bat is £1.05
We'd love to hear your comments and suggestions on this or other brainteasers.