If they had six bags of cookies and each bag has thirteen cookies, then in total they have a total of seventy-eight cookies. If they've eaten twenty-nine cookies, then they now have forty-nine cookies because 78-29=49.
First you want to change the 20% into a decimal which is .20 Next you want to multiply 90 by .20 because that will give you the answer for 20% of 90 90x.20=18