Math Background

Lesson: Justifying Answers
Developing the Concept

Once you have worked through several picture stories with children and had them justify their answers in several ways, introduce written stories and have them justify what they did to find the answer.

Materials: large sheets of paper for demonstration, counters, connecting cubes, beans, or other manipulatives

Preparation: Write stories such as the following on the large sheets of paper.

3 children are eating lunch. 5 more children join them. How many children are eating lunch?

8 children are eating lunch. 3 children finish eating and go out to play. How many children are still eating lunch?

Prerequisite Skills and Concepts: using picture stories to justify answers, addition and subtraction facts to 8

Begin by reading the first story together with the children. Make sure that all children know what the story is about.

  • Ask: What does the problem tell you?
    Children should say that the problem tells us that there are 3 children eating and 5 more children join them to eat lunch.
  • Ask: How can we find the answer? Do you think we should add or subtract?
    Children may suggest different strategies, but will probably say that they can add the numbers of children—3 and 5.
  • If children suggest a strategy, such as drawing a picture, ask a volunteer to show it on the board.
  • Ask: Can someone write the number sentence on the board?
    The child should write 3 + 5 = 8.
  • Ask: Do you think our answer is reasonable?
    Children will probably say yes.
  • Ask: How can we justify our answer? If I try another way, do I get the same answer?
    Allow time for children to suggest strategies. They may suggest drawing a picture, if that wasn't done before, making a table, or using manipulatives. Have children use one of the strategies to justify the answer.
  • Repeat the process, using the second story and subtracting. As the class works through the problem together, encourage the children to talk about what they are thinking as they solve the problem and answer the question. Again, have children choose another way to solve the problem and to justify the answer. Encourage lots of discussion and use lots of positive reinforcement as children suggest different strategies to solve problems and justify the answers. Continue the process with other story problems before having children work in groups or on their own.

Wrap-Up and Assessment Hints
Once children have worked together in class and group discussions, you will want to have them work on their own. As you circulate, ask questions that lead children to reveal their thinking. Continue to emphasize how important it is that children check their work and justify their answers. Encourage them to show their work.

Houghton Mifflin Math Grade 1