Make a Ten Strategy

Calculate $8 + 7$ mentally

Identify how to make 10: $8$ needs $2$ more to make $10$ = $8 + 2 = 10$

Take 2 from the other number: Take $2$ from $7$, leaving $5$ = $7 = 2 + 5$

Add the ten and the rest: $10 + 5$ = $15$

Answer: $8 + 7 = 15$ by making a ten

Break Apart Numbers

Calculate $47 + 36$ mentally

Break apart both numbers: $47 = 40 + 7$ and $36 = 30 + 6$ = Split by place value

Add the tens: $40 + 30 = 70$ = $70$

Add the ones: $7 + 6 = 13$ = $13$

Combine the results: $70 + 13$ = $83$

Answer: $47 + 36 = 83$

Use Friendly Numbers

Calculate $99 + 47$ mentally

Round to a friendly number: $99$ is close to $100$ = Use $100$ instead

Add the friendly number: $100 + 47 = 147$ = $147$

Adjust for rounding: We added $1$ too much ($100$ instead of $99$) = Subtract $1$

Calculate final answer: $147 - 1$ = $146$

Answer: $99 + 47 = 146$

Doubles and Near Doubles

Calculate $7 + 8$ mentally

Find a double you know: $7 + 7 = 14$ (double) = Start with double $7$

Adjust for the difference: $8$ is $1$ more than $7$ = Add $1$ more

Calculate: $14 + 1$ = $15$

Answer: $7 + 8 = 15$ using near doubles

Left to Right Addition

Calculate $256 + 123$ mentally

Start with hundreds: $200 + 100 = 300$ = Keep $300$ in mind

Add the tens: $50 + 20 = 70$, so $300 + 70 = 370$ = $370$

Add the ones: $6 + 3 = 9$, so $370 + 9 = 379$ = $379$

Answer: $256 + 123 = 379$

Mistake: Forgetting to adjust after using friendly numbers

Why: When you round $99$ to $100$, you added $1$ extra. If you forget to subtract that $1$, your answer will be off.

Correct: Always track what you added or subtracted when rounding, then adjust at the end.

Mistake: Mixing up place values when breaking apart

Why: Writing $47$ as $4 + 7$ instead of $40 + 7$ ignores place value completely.

Correct: $47 = 40 + 7$, not $4 + 7$. The $4$ represents $4$ tens, which is $40$.

Mistake: Trying to do too many steps at once

Why: Mental math works best when you break problems into small, manageable pieces.

Correct: Take it step by step. It's okay to pause and hold a number in your head before continuing.

Shopping Without a Calculator

Quickly add prices in your head while shopping to stay within budget.

A book costs 12 dollars and a notebook costs 8 dollars. Using make-a-ten: $12 + 8 = 20$ dollars.

Keeping Game Scores

In board games or sports, mental math helps you track scores quickly.

Your team scored 28 points in the first half and 35 in the second. Using break apart: $28 + 35 = 20 + 30 + 8 + 5 = 50 + 13 = 63$ points.

Time Calculations

Figure out when something will happen or how long something takes.

It's 45 minutes past the hour, and you need 25 more minutes. $45 + 25 = 45 + 15 + 10 = 60 + 10 = 70$ minutes, or 1 hour and 10 minutes.

Q: Which strategy should I use?

A: It depends on the numbers! For numbers close to 10 or 100, use friendly numbers. For single digits, try making tens or doubles. For larger numbers, break apart by place value. Practice will help you pick the best strategy quickly.

Q: What if I forget a number in the middle of calculating?

A: This is normal when starting out! Try saying the number quietly to yourself, or tap your fingers to keep track. With practice, holding numbers in memory gets easier.

Q: Is mental math faster than using a calculator?

A: For simple problems, yes! By the time you pull out a calculator and type the numbers, you could already have the answer in your head. Plus, mental math strengthens your brain!