Mental adding and subtracting
I can add and subtract ones, tens and hundreds in my head.
For grown-ups: this lesson in words
Years 3 to 6 · Maths
Mental adding and subtracting: I can add and subtract ones, tens and hundreds in my head.
346 + 20. Think: 20 is 2 tens, so hop 10 two times. Start at 346.
Hop 10: 346 + 10 = 356.
Hop 10 again: 356 + 10 = 366.
346 + 20 = 366. Only the tens digit changed.
Picture: A number line from 340 to 370, labelled every 10. A dot at the 6th tick after 340 labelled "start". The frog hops from the 6th tick after 340 to the 6th tick after 350 (+10), then to the 6th tick after 360 (+10).
Now subtract hundreds. 628 − 300: start at 628.
Hop back 100 three times: 528, 428, 328.
628 − 300 = 328. Only the hundreds digit changed.
Picture: A number line from 300 to 700 with a tick every 20, labelled every 100. A dot at a point between the 1st and 2nd ticks after 600 labelled "start". The frog hops from a point between the 1st and 2nd ticks after 600 to a point between the 1st and 2nd ticks after 500 (−100), then to a point between the 1st and 2nd ticks after 400 (−100), then to a point between the 1st and 2nd ticks after 300 (−100).
Your turn: 254 + 30. Start at 254 and hop 10 three times. What do you get?
Picture: A number line from 250 to 285, labelled every 10. A dot at the 4th tick after 250 labelled "start".
Choose one:
- 257
- 284
- 554
Answer
The answer: 284
254 + 30 = 284. 264, 274, 284.
Picture: A number line from 250 to 285, labelled every 10. A dot at the 4th tick after 250 labelled "start". The frog hops from the 4th tick after 250 to the 4th tick after 260 (+10), then to the 4th tick after 270 (+10), then to the 4th tick after 280 (+10).