Students researching backpack weights gathered data from 45 elementary school children in the 3rd and 5th grades. the variable is “percent of body weight carried in the school backpack.” so a child who weighs 60 pounds and carries 9 pounds has a variable value of 15% (9 ÷ 60 = 0.15 = 15%). the american chiropractic association (aca) recommends that children carry no more than 10% of their body weight.