The expression is 

Place value chart for 7.80 and 3.30

.

 Line up the decimal points and draw a line below the numbers.

First find the difference of the hundredths digit.

The difference of the hundredths digit = .

First find the difference of the tenths digit.

The difference of the tenths digit = .

Next find the difference of the ones digit.

The difference of the ones digit = .

The amount left with Calvin is