Strategies
Make a 10
The use of this strategy is based on knowing what single-digit numbers add to 10: 1+9, 2+8, 3+7, 4+6, 5+5. The idea is to add or subtract an amount from one number to make it a multiple of 10, then subtract or add that amount to the other number to complete the addition.
Example: 8+5 = (8+2) + (5-2) = 10 + 3 = 13 (we choose to Make a 10 by adding 2 and subtracting 2)
Look For Doubles
The use of this strategy is based on being able to recognize doubles and knowing what the value of a double is.
Example: 5+5 = 10
Count On
The use of this strategy is based on being able to count the number of steps indicated by one of the numbers.
Example: 8+5 = 8 counted up by 5. We count 9, 10, 11, 12, 13 and recognize that when we get to 13, we have added 5. (I like this strategy when counts of only 1, 2, or 3, or occasionally, 4 are involved.)
Doubles Plus 1
Similar to the Look For Doubles strategy, this strategy depends on being able to recognize when the sum of a pair of numbers differs from a double by 1.
Example: 5+6 = 5 + (5+1) = (5+5) + 1 = 10 + 1 = 11
Look for Doubles (extended)
The Doubles Plus 1 strategy is a sort of combination of the Look For Doubles strategy and the Count On strategy. This sort of extension can be carried further.
Example: 8+5 = (5+3) + 5 = (5+5) + 3 = 10 + 3 = 13 (note that we have added more than 1 to the double.)
Adding 3 Numbers
Addition is a "binary" operation in that it can only be done two numbers at a time. So, when the problem is adding 3 numbers, any two can be added first, then the third added to the sum. This often gives the opportunity to choose the pair of numbers you want to start with such that you can apply the easiest addition strategy or strategies to get the sum.
All of these strategies are tools to use to help you do math faster and more accurately in your head. If you always wear a calculator watch, then perhaps you don't need them. For the rest of us, they come in handy on occasion.
Make a 10
The use of this strategy is based on knowing what single-digit numbers add to 10: 1+9, 2+8, 3+7, 4+6, 5+5. The idea is to add or subtract an amount from one number to make it a multiple of 10, then subtract or add that amount to the other number to complete the addition.
Example: 8+5 = (8+2) + (5-2) = 10 + 3 = 13 (we choose to Make a 10 by adding 2 and subtracting 2)
Look For Doubles
The use of this strategy is based on being able to recognize doubles and knowing what the value of a double is.
Example: 5+5 = 10
Count On
The use of this strategy is based on being able to count the number of steps indicated by one of the numbers.
Example: 8+5 = 8 counted up by 5. We count 9, 10, 11, 12, 13 and recognize that when we get to 13, we have added 5. (I like this strategy when counts of only 1, 2, or 3, or occasionally, 4 are involved.)
Doubles Plus 1
Similar to the Look For Doubles strategy, this strategy depends on being able to recognize when the sum of a pair of numbers differs from a double by 1.
Example: 5+6 = 5 + (5+1) = (5+5) + 1 = 10 + 1 = 11
Look for Doubles (extended)
The Doubles Plus 1 strategy is a sort of combination of the Look For Doubles strategy and the Count On strategy. This sort of extension can be carried further.
Example: 8+5 = (5+3) + 5 = (5+5) + 3 = 10 + 3 = 13 (note that we have added more than 1 to the double.)
Adding 3 Numbers
Addition is a "binary" operation in that it can only be done two numbers at a time. So, when the problem is adding 3 numbers, any two can be added first, then the third added to the sum. This often gives the opportunity to choose the pair of numbers you want to start with such that you can apply the easiest addition strategy or strategies to get the sum.
All of these strategies are tools to use to help you do math faster and more accurately in your head. If you always wear a calculator watch, then perhaps you don't need them. For the rest of us, they come in handy on occasion.