Hanna has 34 pennies and Mandy 48.
If we add the pennies they have we get:
The mistake is that Mandy got a different result than the real addition of their pennies.
Her result was 72, but they have 82 pennies.
To fix her mistake she shoud have an 8 instead of a 7 in the tens place, so she should add 10 to his result.