First, students will learn how to add within 20. One method, is to add by counting forwards, starting from the first addend. Another method, is to add by making tens. For example, 5 + 9 = 4 + 10 = 14.

Watch these Khan Academy videos:

Do these Khan Academy exercises:

Next, students will learn how to subtract within 20. One method, is to subtract by counting backwards, starting from the minuend. Another method, is to subtract by making tens. For example, 13 - 4 = 10 - 1 = 9.

Watch these Khan Academy videos:

Do these Khan Academy exercises:

This standard is too vague. Exactly which methods should they be using? For example, can they add and subtract with a number line, or is that not considered fluent?

Next, give your students these challenges:

- 2017 Math Kangaroo Levels 1-2 Problem #18 by STEM4all

Make the equation below true by replacing each letter with a unique digit (0-9).

$$W + O = OF$$Here's the solution:

The largest sum of two digits is \(9 + 9 = 18,\) so \(O = 1.\) Now we have \(W + 1 = 1F.\) The only way to make \(W + 1 \ge 10\) is if \(W = 9.\) Now we have \(9 + 1 = 1F,\) so \(F = 0.\) In conclusion, the equation is \(9 + 1 = 10.\) Note that it doesn't matter whether we accept leading zeros, such as \(05,\) because if \(O = 0,\) then \(W = F,\) which violates the condition that \(W\) and \(F\) must be unique.

Conclude by leading this investigation:

Raindrops Down Windshield (number order, algorithm)

by MathPickle

1.OA.C.6: Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 - 4 = 13 - 3 - 1 = 10 - 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 - 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).

1.OA.C.5: Relate counting to addition and subtraction (e.g., by counting on 2 to add 2).