Lesson 7 Round Doubloons

    • Let’s explore rounding with decimals.

Warm-up Notice and Wonder: A Digital Scale

What do you notice? What do you wonder?

12 and 3 tenths ounces on scale screen. Weights include. 10 ounce weight, 1. 1 ounce weights, 2, 1 tenth ounce weights, 3, 1 hundredth ounce weights, 2.  

Activity 1 Gold Doubloons

Problem 1

image of front and back of the doubloon coin.

  • Until 1728, doubloons weighed 6.867 grams.

  • After 1728, they weighed 6.766 grams.

  1. You have a scale that measures weight to the nearest tenth of a gram.

    Was the doubloon on the scale made before or after 1728?

    Image of a doubloon on a scale showing 6 and 8 tenths gram.

  2. If you had a scale that measured to the nearest gram, explain why you would not be able to tell what year the doubloon was made based on the weight listed on the scale.

  3. Which doubloons weigh more, the ones made before 1728 or the ones made after 1728? Explain or show your reasoning.

  4. Show the weights of the doubloons on the number line.

    Number line. Scale, 6 and 7 tenths to 6 and 9 tenths, by tenths. 

Problem 2

Use the number lines to find which hundredth of a gram the doubloon weights are each closest to.

  1. Number line. Eleven evenly spaced tick marks. First tick mark, 6 and 76 hundredths. Last tick mark, 6 and 77 hundredths.
  2. Number line. Eleven evenly spaced tick marks. First tick mark, 6 and 86 hundredths. Last tick mark, 6 and 87 hundredths.

Activity 2 Exact or Approximate?

Decide if each quantity is exact or an estimate. Be prepared to explain your reasoning.

  1. There are 14 pencils on the desk.

  2. The population of Los Angeles is 12,400,000.

  3. It’s 2.4 miles from the school to the park.

  4. The runner finished the race in 19.78 seconds.

Practice Problem

Problem 1

A $5 gold coin weighs 8.359 grams.

  1. Locate 8.359 on the number line.

  2. A scale measures to the nearest 0.01 gram. What will the scale show for the weight of the coin? Explain or show your reasoning.