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In this lesson we add and subtract whole tens, such as 50 + 40 or 90 − 70, using mental math. (These types of numbers are also called multiples of ten in the higher grade levels; I just don't use that terminology when talking with 1st grade students.)

First, students can use pictures (base ten blocks) to add and subtract, but I'm sure they soon notice the SHORTCUT: you can simply work with the number of tens the same way you add or subtract single-digit numbers. That way, 50 + 40 becomes 5 tens + 4 tens, which is 9 tens.

Lastly, Mathy gives us a challenging addition problem to solve... making TENS helps a lot!

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Two-digit mental addition — online practice

Math Mammoth Place Value 1 — workbook for 1st grade math

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