Numbers like 20, 30 and 40 are whole tens. To add them, add the tens just like ones. 3 tens and 4 tens make 7 tens, so 30 + 40 = 70.
This works because each ten is a group of ten ones. Thinking in tens makes big sums small.
You can use facts you already know. Because 2 + 5 = 7, you also know that 20 + 50 = 70. The tens behave just like ones.
Worked example
50 + 30 =
- 50 is 5 tens and 30 is 3 tens.
- 5 tens + 3 tens = 8 tens.
Answer:80
Key words
- ten
- a group of 10 ones
- whole tens
- numbers that end in 0, like 60
- tens fact
- a fact like 4 + 3 = 7 used as 40 + 30 = 70
Practise this
New questions every time.
Common Core-referenced: 1.NBT.C.4