After elsa backed muffins, her brother ate 4 of them, he ate 2/5 of all the muffins, how many muffins did elsa bake?

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