Algebraic Expressions & Equations

Weight in Pounds

Work Time

Weight in Pounds

Many people in the United States know their weight only in pounds. There is a rule of thumb that can help you use your weight in pounds to estimate your weight in kilograms: divide your weight in pounds by 2, and then subtract 10% of that number from the result.

  • Let p = a person’s weight in pounds. Write an expression for the person’s weight in kilograms.
  • Can you write an equivalent expression for the person’s weight in kilograms?
  • Use the rule of thumb to find the weight, in kilograms, of a person who weighs 115 lb.
  • Use the rule of thumb to find the weight, in pounds, of a person who weighs 49.5 kg.

Hint:

  • Equivalent expressions are expressions that have the same value when a given value is substituted for the variable. For example, 3(x + 6) and 3x + 18 are equivalent expressions. Whenx = 2, both expressions have the value 24.
  • If you have 80 pounds, how do you take 10% of it?
  • How might you use the distributive property to write equivalent expressions?
  • Now think back to the difference between an expression and an equation. To find a person’s weight in kilograms, do you need to evaluate an expression or solve an equation?
  • To find a person’s weight in pounds, do you need to evaluate an expression or solve an equation?