Chapter 4.5, Problem 9P

Here is Marco’s method for calculating 38 60: Four times 6 is 24, so 40 times 60 is 2400. Then 2 times 6 is 12, so it’s $2400—120,$ which is 2280.

Write equations that incorporate Marco’s method and that also show why his method is valid. Write your equations in the following form:

   $\begin{array}{l}38.60=someexpression\hfill \\ =someexpression\hfill \\ \begin{array}{l}=:\\ =2280\end{array}\hfill \end{array}$

Which properties of arithmetic did Marco use (knowingly or not), and where?