Multiply 2 Two-Digit Numbers

10 min

Narrative

This Number Talk encourages students to use multiples of 10 to mentally multiply two-digit numbers that are close to multiples of 10. Students can use place value reasoning and the distributive property of multiplication over addition or subtraction to find the value of the products. Students think flexibly about how numbers can be decomposed strategically when multiplying.

Launch

  • Display one expression.
  • “Give me a signal when you have an answer and can explain how you got it.”
  • 1 minute: quiet think time
Teacher Instructions
  • Record answers and strategy. 
  • Keep expressions and work displayed.
  • Repeat with each expression.

Student Task

Find the value of each expression mentally. 

  • 20×6020 \times 60
  • 21×6021 \times 60
  • 20×6220 \times 62
  • 19×6019 \times 60

Sample Response

  • 1,200: I know 2×62 \times 6 is 12, so 2×10×6×102 \times 10 \times 6 \times 10 is 12×10012 \times 100, which is 1,200.
  • 1,260: 21×6021 \times 60 is one more group of 60 than 20×6020 \times 60, so it is 1,200+601,200 + 60 or 1,260 (or (20×60)+(1×60)=1,260(20 \times 60) + (1 \times 60)= 1,260).
  • 1,240: 20×6220 \times 62 is 20×220 \times 2 more than 20×6020 \times 60, so it is 40 more than 1,200 (or (20×60)+(20×2)=1,240(20 \times 60) + (20 \times 2) = 1,240).
  • 1,140: 19 groups of 60 is 1 group of 60 less than 20 groups of 60, so it is 1,200–601,200 – 60 or 1,140 (or (20×60)−(1×60)=1,140(20 \times 60) - (1 \times 60) = 1,140).
Activity Synthesis (Teacher Notes)
  • “How can 20×6020 \times 60 help us find the value of 19×6019 \times 60?” (19×6019 \times 60 is one group of 60 less than 20×6020 \times 60, so we can subtract 60 from 20×6020 \times 60.)
Standards
Building Toward
  • 4.NBT.5·Multiply a whole number of up to four digits by a one-digit whole number, and multiply two two-digit numbers, using strategies based on place value and the properties of operations. Illustrate and explain the calculation by using equations, rectangular arrays, and/or area models.
  • 4.NBT.B.5·Multiply a whole number of up to four digits by a one-digit whole number, and multiply two two-digit numbers, using strategies based on place value and the properties of operations. Illustrate and explain the calculation by using equations, rectangular arrays, and/or area models.

20 min

15 min