Three- and four-digit numbers · Usually met around P2
Odd and even in addition
Odd + odd = even, odd + even = odd, even + even = even.
How to do it
Method
Odd + odd = even, odd + even = odd, even + even = even
347 + 125: odd + odd, so the sum is even (472)
Another way
Pair the dots
3 + 5: one left over each, and the two pair up
In full
An even number pairs up exactly; an odd number has 1 left over. When you add two numbers, look at what's left over: even + even leaves nothing, so the sum is even; odd + even leaves just the odd number's 1, so it is odd; odd + odd leaves two 1s, which pair up, so it is even. So without working out the sum, the two ones digits tell you whether it is odd or even. Or lay the dots out in two rows and see whether one is left on its own.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
Without working them out: which of these sums is odd?
- A. 914 + 132
- B. 411 + 965
- C. 569 + 746
- D. 567 + 917
Choose the right answer
Show the answer and working
Answer: C. 569 + 746
569 ends in an odd digit and 746 in an even one: odd + even = odd.
Example 2
Without working it out: 950 + 591 is an odd number.
True or false?
Show the answer and working
Answer: True
Look at the ones digits only: odd plus even is odd; two odds or two evens make an even number. 950 + 591 = 1541.
Example 3
From 6 to 15 (counting both), how many numbers give an even number when 9 is added?
Show the answer and working
Answer: 5
To give an even sum with 9, a number must be odd if 9 is odd and even if 9 is even: from 6 to 15 there are 5 of them.
Practise it in the game →Three- and four-digit numbers · Free to start, unlimited questions, no download