options: Explain why addition and subtraction strategies work, using place value and the properties of operations. (Explanations may be supported by drawings or objects.)

Add up to four two-digit numbers using strategies based on place value and properties of operations.

Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method. Understand that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.

Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.
problem_activity: Task
Materials
Popsicle sticks and rubber bands or base-10 blocks
Paper and pencil for each student
Actions
The teacher should pose the following question to students:
Louis wants to give \$15 to help kids who need school supplies. He also wants to buy a pair of shoes for \$39. If Louis gets \$1 every day for his allowance, how many days will it take him to save enough money for both? Explain how you know.


correct_option_index: 2