Math Games

Grade 3 - Number

Standard 3.N.2 - Practice subtracting half from a number. Recognize the similar numbers in subtracting half of a number.

Included Skills:

Demonstrate understanding of addition of whole numbers with answers to 1000 and their corresponding subtractions (limited to 1, 2, and 3-digit numerals) including:
• representing strategies for adding and subtracting concretely, pictorially, and symbolically
• solving situational questions involving addition and subtraction
• estimating using personal strategies for adding and subtracting.
• Describe personal mental mathematics strategies that could be used to determine a given basic fact, such as:
- doubles (e.g., for 6 + 8, think 7 + 7)
- doubles plus one (e.g., for 6 + 7, think 6 + 6 + 1)
- doubles take away one (e.g., for 6 + 7, think 7 + 7 - 1)
- doubles plus two (e.g., for 6 + 8, think 6 + 6 + 2)
- doubles take away two (e.g., for 6 + 8, think 8 + 8 - 2)
- making 10 (e.g., for 6 + 8, think 6 + 4 + 4 or 8 + 2 + 4)
- commutative property (e.g., for 3 + 9, think 9 + 3)
- addition to subtraction (e.g., for 13 - 7, think 7 + ? = 13)
• Observe and generalize personal strategies from different types of representations for adding 2-digit quantities (given concrete materials, pictures, and symbolic decompositions) such as:
- Adding from left to right (e.g., for 23 + 46 think 20 + 40 and 3 + 6)
- Taking one or both addends to the nearest multiple of 5 or 10 (e.g., for 28 + 47, think 30 + 47 - 2, 50 + 28 - 3, or 30 + 50 - 2 - 3)
- Using doubles (e.g., for 24 + 26, think 25 + 25, or for 25 + 26, think 25 + 25 + 1).
• Observe and generalize personal strategies for subtracting 2-digit quantities (given concrete materials, pictures, and symbolic decompositions) such as:
- Taking the subtrahends to the nearest multiple or 10 (e.g., for 48 - 19, think 48 - 20 + 1)
- Thinking of addition (e.g., 62 - 45, think 45 + 5, 50 + 12 to get from 45 to 62, so the difference is 5 + 12)
- Using doubles (e.g., for 25 - 12, think 12 + 12 = 24 and 24 is one less than 25, so difference is 12 + 1).
• Apply and explain personal mental mathematics strategies to determine the sums and differences of two-digit quantities.
• Create a situational question that involves either addition or subtraction and that has a given quantity as the solution.
• Model (concretely or pictorially) a process for the addition of two or more given quantities (with a sum less than 1000) and record the process symbolically.
• Model (concretely or pictorially) a process for the subtraction of two or more quantities (less than 1000) and record the process symbolically.
• Generalize (orally, in writing, concretely, or pictorially) personal strategies for estimating the sum or difference of two 2-digit quantities.
• Extend personal mental mathematics strategies to determine sums and differences (of quantities less than 1000) and explain the reasoning used.
• Transfer knowledge of the basic addition facts up to 18 and the related subtraction facts to determine the sums and differences of quantities less than 1000.
• Generalize rules for the addition and subtraction of zero.
• Provide examples to show why knowing about place value is useful when adding and subtracting quantities.