Math Mind Magic
Students will be able to mentally find 10 more or 10 less than a given two-digit number without counting.
Introduction (10 minutes)
- Call students together around the hundreds chart.
- If you have wardrobe items, wear them.
- Tell your students that today they are going to be math magicians. In order to perform this magic trick, inform them that they will need some props.
- Hand out lots of index cards to each student.
- Instruct your students to write a number between 10-90 on one side of each card. On the other side of the cards, have the students write a plus sign with the number 10 and a minus sign with the number 10.
- Demonstrate this process for students. Explain that they can count or use the hundreds chart to figure out what number is 10 greater (more) or 10 less (smaller).
Explicit Instruction/Teacher Modeling (10 minutes)
- Ask a student to choose any of her cards, and tell her that you will immediately tell her the number ten greater and ten less without counting or using a chart. Make sure to have your back to the hundreds chart.
- Once you have successfully completed the magic trick a few times, ask students how they think you did it.
- After students guess, explain to students that all they need to do is add or subtract one from the number in the tens place.
- Ask students to think about why it only takes adding or subtracting one from the tens place. Explain that when they go up or down by one in the tens place, they are really going up or down by ten ones.
- Show students the unit blocks. Demonstrate that when they stack ten single blocks up, it is the same as the tens stick. Show students a number, such as 27, and demonstrate that taking away ten single blocks is the same as taking away one tens stick.
- As another visual to help students understand why the magic trick works, have them look at the hundreds chart.
- Count 10 squares from any given number. Demonstrate that this is just the number one row above or below. Once again, the tens place value just changes by one.
Guided Practice/Interactive Modeling (5 minutes)
- Call on several students to attempt the magic trick.
- When they are done announcing what is ten more and ten less, have them explain in their own words what they did and why it works.
- Explain to students that they will now have a chance to practice with partners.
- Have them take turns being the magician and the prompter using the flashcards they created earlier in the lesson.
- Direct students to record their answers on sheets of paper.
- Ask them to explain what they did and why it works on their papers with the answers.
Independent Working Time (10 minutes)
- Have your students carry their cards and walk around, saying the answers quickly as they see a card. Encourage your students to walk around the room a lot so that multiple people can see the cards and respond.
- As students are working, circulate the room and check for understanding.
- Enrichment: For students who need a greater challenge, including decimal points or higher place values can increase the difficulty.
- Support: For students who need a little extra assistance, having personal hundreds charts to reference in the beginning can be useful. Working in partners initially can also help to scaffold the lesson.
Related Books and/or Media
Assessment (5 minutes)
- Assess your students based on their proficiency and accuracy in performing the “magic trick” as documented on the sheets of paper they recorded their independent work time answers on.
Review and Closing (5 minutes)
- If you are including the wardrobe items, tell your students that you are going to pass on the job of magician.
- Ask several students to volunteer to dress up and perform the magic act.
- Have your students call out numbers, and have the magicians name the number 10 greater and 10 fewer.
- After the student magicians have performed, have them explain their thought processes and how they came up with their answers to the class.
- Remind students that to make a number greater by ten, all they have to do is add one to the tens digit, and to make a number smaller by ten, they just have to subtract one from the tens digit.