Mary removed the dollar sign from (X.)
We know this because $4.00x$5.00=$20.00 so that’s correct and we know that $12.50x $4.00=$50.00. So, our only error here is that there is no dollar sign in our (X) numbers. So that is Mary’s mistake. We can correct it by simply adding a dollar sign to 20 to make $20.00 and adding another dollar sign to 50 to make $50.00
Hope this helps!