If he ate 4 of them, but it means he ate 2/5 muffins,
4 muffins = 2/5 muffins
You substitute x muffins for how much muffins she baked in all -
2/5* x muffins = 4
2/5 multiply by 2/5 to cross it out. Do the same to the other side.
x= 4 *5/2
x = 10 muffins
So there were 10 muffins in all