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Jar of jellybeans

By David, 24 March 2015 Activity

It’s a tempting treat and a mathematical conundrum. We’ll show you how to run a jellybean guessing competition, and give you some hints on how to win!
food safety hazard iconSafety: When playing with food, use clean hands and clean equipment.

someoen is pouring jellybeans into a jar.

Fill the jar with jellybeans

You will need

  • Lots of jellybeans
  • Large jar
  • Tape measure
  • Scales
  • Cup measure
  • Pens and paper
  • Gloves for jellybean handling
    A tape measure next to a jar of jellybeans.

    Use a tape measure to help estimate the volume of the jar.

  • Lots of people to have a go

Setting up the jar of jellybeans

  1. Fill the jar with jellybeans.
  2. Tightly put the lid on the jar.
  3. The aim of this activity is to estimate how many jellybeans there are in the jar.
  4. The person who gets closest wins the whole jar of jellybeans!

    a jar of jellybeans on a set of scales.

    Remember that you’re measuring the mass of the jellybeans AND the jar at the same time!

Running the challenge

  1. To get everyone started, give out some pens and paper to write on.
  2. Pass the jar around so that everyone gets a chance to hold it and make some measurements.
  3. Have a cup of jellybeans, a tape measure and some scales that everyone can use to make their estimate.
  4. Once everyone has made an estimate, ask them to write it and their name on a piece of paper and hand it in.
  5. Once everyone’s estimates are collected, write them all out on a whiteboard, and then start counting the jellybeans (wear some gloves so the jellybeans can be eaten afterwards). To make sure you don’t lose count, group the beans into piles of 50.
  6. When you finish counting, work out the difference between each estimate and the real answer. The smallest difference (also the best estimate) wins!
  7. If you win, make sure you don’t eat all the beans by yourself!

Estimation techniques

There are lots of ways to estimate the number of jellybeans. Here are some ideas to get you started.

The layer method

Imagine the beans come in layers. Count the beans in the bottom layer of the jar, and then count how many layers would fit in the jar. Multiply these together to get an estimate.

The volume method

Measure one cup of jellybeans and then count them. Measure the outside of the jar and calculate the volume in cups. Then multiply to get your estimate.

The mass method

Weigh out 50g of beans and count them. Weigh the jar of jellybeans, and estimate and subtract the mass of the glass jar to get the mass of jellybeans. With these numbers you should be able to estimate the number of jellybeans in the jar.

The averaging method

Come up with a few different estimates using different methods. Then take an average of those estimates to get a final estimate.

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