Answer :

iGreen
Set up a proportion:

5/7/10 = 1/x

Cross multiply:

5/7x = 10

Divide 5/7 to both sides or multiply by its reciprocal, 7/5:

x = 10 * 7/5

x = 70/5

x = 14

So there were 14 muffins, now subtract 10 from it to find out how many are left.

14 - 10 = 4

So there are 4 muffins left.
Hippalectryon
10 muffins is 5/7 of your muffins.

You had 7/7 of your muffins at the start, which is 7/5 times more.

Hence you had 10*7/5=14 muffins at the beginning.

Thus you have 14-10=4 muffins left !

Shame on that hungry elf └༼ ಥ ᗜ ಥ ༽┘