Math Background

Lesson: Justifying Answers
Introducing the Concept

Your children may have encountered problems interpreting pictures prior to reading and solving word problems. You may want to begin by having children work through and discuss picture stories before going on to reading story problems. The most important part of the lesson should be to show children how to use different strategies for justifying their answers.

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

Preparation: Prepare several picture story problems such as shown here.

3 children waiting at bus stop are joined by a fourth child.
3 children flying 3 kites. 1 kite's string breaks and flies away.

Prerequisite Skills and Concepts: Children should know how to add and subtract facts to 6.

Begin by displaying the first picture on the board.

  • Ask: Who can tell me what is happening in the first picture?
    Children should say that there are 3 children at the bus stop, and 1 more child is running to join the group.
  • Ask: Who can tell me what is happening in the second picture?
    Children should say that now there are 4 children at the bus stop.
  • Ask: Does this picture show addition or subtraction? Why?
    Children will probably say addition, because they see that another child is joining the group.
  • Say: That is correct. When something is joined, we add.
  • Ask: Who would like to tell us the number sentence and write it on the board?
    The child should write 3 + 1 = 4.
  • Ask: Is the answer reasonable? Children will probably answer yes.
  • Ask: Is there a way that we can prove that the answer is correct?
    Lead children to see that one way to justify the answer is to try the problem in another way, perhaps using counters. Have children work through the problem, using the suggested manipulative or another strategy.
  • Say: We found the answer to the problem, and then we solved the problem another way. We came up with the same answer, so we know that the answer is correct. We say that we justified the answer.
  • You may want to ask if the children can suggest another way to prove that the answer is correct. You might suggest a simple table showing on one line the 3 children at the bus stop and on the second line, 1 child running. Then have children count how many in all. The answer is still the same.
  • Repeat the process using the second picture and solving the problem with subtraction. Remind children that when things are separated, or go away, they are subtracted.
Houghton Mifflin Math Grade 1