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.