A brand of cereal had 1.21.21, point, 2 milligrams (\text{mg})(mg)left parenthesis, m, g, right parenthesis of iron per serving. Then they changed their recipe so they had 1.8\,\text{mg}1.8mg1, point, 8, space, m, g of iron per serving. What was the percent increase in iron

What you can do is solve this problem by a rule of three to know the result.
 Making the rule of three we have:
 1.21 ---> 100%
 1.8 ---> x
 Clearing x:
 x = (1.8 / 1.21) * 100 = 148.76
 Then, the percentage of increase is:
 148.76-100 = 48.76%
 answer
 the percent increase in iron was 48.76%