Plan a Party
Estimate costs, guests, food, and supplies.
Algebra Readiness · Unit 1
Mental Addition and Subtraction with Two-Digit Numbers
I canadd and subtract two-digit numbers mentally.
Mental math does not mean guessing or solving instantly. It means changing numbers into parts that are easier to work with in your head.
Learn the Strategies46 + 28
+2066+874
One problem. Useful mental steps.Why should I care?
Mental Math Helps You Make Quick Decisions
You may not have paper ready when you are planning an event, checking a budget, counting supplies, or deciding whether you have enough.
Manage a Team
Check uniform costs and remaining money.
Work an Event
Track tickets, wristbands, and merchandise.
Organize Supplies
Know what arrived, what was used, and what remains.
Start here
See the Tens and Ones Inside a Number
Two-digit numbers are made of tens and ones. Seeing those parts gives your brain easier numbers to use.
Tens440
+Ones77
=47Important: You do not have to picture blocks every time. The visual helps you understand why breaking apart numbers works.
1
Mental strategy
Break Apart Tens and Ones
I can separate each number into tens and ones.
47 + 26
1
40 + 20 = 60
2
7 + 6 = 13
3
60 + 13 = 73
TRY IT NOW
Break Apart Practice
🎉 52 guests + 34 guests
Party guest list52 + 34
Two groups are coming to the party. How many guests are expected altogether?
2
Mental strategy
Add or Subtract in Chunks
I can keep one number whole and move in useful chunks.
68 − 24
1
68 − 20 = 48
2
48 − 4 = 44
TRY IT NOW
Useful Chunks Practice
📦 76 boxes − 31 boxes
Delivery center76 − 31
A delivery center had 76 boxes and loaded 31 onto a truck. How many are still inside?
3
Mental strategy
Make a Friendly Number
I can change a number to one that is easier to use.
38 + 27
1
38 needs 2 to reach 40
2
Move 2 from 27 to 38
3
40 + 25 = 65
TRY IT NOW
Friendly Number Practice
🎈 59 blue + 24 gold
Party decorations59 + 24
How many blue and gold balloons are ready for the party?
4
Mental strategy
Adjust and Compensate
I can round one number, solve, and correct my change.
62 − 29
1
Think: 62 − 30 = 32
2
We removed 1 too many
3
32 + 1 = 33
TRY IT NOW
Adjust and Compensate Practice
🥤 81 drinks − 39 used
Party drink station81 − 39
How many drinks remain after guests use 39?
5
Mental strategy
Keep the Same Difference
I can move both numbers the same amount to make subtraction easier.
74 − 38
1
Move both numbers up 2
2
76 − 40 = 36
TRY IT NOW
Same Difference Practice
💳 $67 budget − $29 cost
Uniform budget67 − 29
How much of the uniform budget remains?
Which strategy should I use?
Let the Numbers Help You Decide
There is not one required mental strategy. Look for a useful feature in the numbers.
Remember
Being good at mental math does not mean being the fastest person. It means understanding numbers well enough to choose a useful strategy and explain your thinking.