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Lesson Plan
Illustrative Mathematics

Illustrative Mathematics: Boys and Girls, Variation 2

For Teachers 1st - 2nd Standards
Learners are asked to solve a word problem with both addends unknown. They will write an equation and draw a picture to use to explain their answer.
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Activity
Wolfram Research

Wolfram Math World: Addition

For Students 6th - 8th Standards
MathWorld provides a detailed definition of addition.