Problem $3-21$ Profit Margin $[$LO$4]$

In response to complaints about high prices, a grocery chain runs the following advertising campaign: $"$If you pay your child $$2$ to go

buy $$40$ worth of groceries, then your child makes twice as much on the trip as we do$."$ You've collected the following information from

the grocery chain's financial statements:

a$.$ What is the profit margin for the child as a percentage of what they spend and the profit margin for the store?

Note: Do not round intermediate calculations and enter your answers as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.$

b$.$ What is the store's ROE?

Note: Do not round intermediate calculations and enter your answer as a percent rounded to $2$ decimal places, e$.$g$.,32\mathrm{.}16.$