4 × 30 means 4 groups of 3 tens. That is 12 tens.
Step 1: Let's Learn
Read it, or press Listen and follow the words.
12 tens is 120
So 4 × 30 = 120. The zero is there because you counted tens, not ones.
Not just "add a zero"
The zero appears because the unit is a ten. Saying "add a zero" hides that.
The quick way
Multiply the two digits, then say the answer in tens.
Multiply the digits, then count the zeros
4 × 30 is 4 × 3 = 12, then one zero from the 30, giving 120. You are really doing 4 × 3 tens = 12 tens = 120. The zero-counting shortcut is fine as long as you can say what it means.
Why it works
30 is 3 × 10, so 4 × 30 is 4 × 3 × 10. The associative property lets you group it as (4 × 3) × 10 = 12 × 10 = 120. This is the property from the last lesson doing something immediately useful.
It scales up
4 × 300 is 12 with two zeros: 1,200. And 40 × 30 is 12 with two zeros: 1,200. Count all the zeros from both numbers. This one rule handles a huge range of calculations that would otherwise need the column method.
Step 2: Try It Yourself
Tap and try it out.
4 × 3 = 12
4 rows of 3 makes 12.
Step 3: Watch an Example
One step at a time.
Watch Leo Count in Tens
Leo is asked for 6 × 40.
- Step 1
He reads 40 as 4 tens.
Step 4: Your Turn
Practice makes it stick.
The Boxes
Problem 1 of 2
5 boxes each hold 20 pencils. How many pencils?
pencils
The Seats
Problem 2 of 2
7 rows of 30 seats. How many seats?
seats
Times a Multiple of Ten
1 of 4
3 × 40 = ?
2 of 4
8 × 50 = ?
3 of 4
9 × 80 = ?
4 of 4
6 × 60 = ?
Step 5: Quick Check
Show what you know.
Question 1 of 2
7 × 40 = ?
Question 2 of 2
Why does 4 × 30 end in a zero?
What You Learned
- 4 × 30 is 4 groups of 3 tens.
- Multiply the digits, then say the answer in tens.
- The zero appears because the unit is a ten.