It is good not to erase too much, so that students can see work done previously. For example, the expression

 

69÷3/4.

 

They noted that this is the same as

 

69÷ .75.

 

This is fine, and this expression was used. The first expression should not be erased, as it is also fine, and can be used, by multiplying by the reciprocal.

 

We may also tell them that ¼ = 25¢, as there are four quarters in a dollar.

 

M kept erasing the board. I suggested he try to keep the old stuff visible as much as possible, so that the students can refer back to it. At universities, classes have large numbers of blackboards that slide over each other.