Liri had changed since she had started doing sums with Erklärus. Not her shape – but her questions were getting bigger.
Until now you knew numbers up to 20. Now the number line goes on – in steps of ten: 10, 20, 30 … up to 100. Every multiple of ten is like a stop on a long journey.
A spaceship collects rock samples: 34 from one asteroid, 25 from another. How many altogether? There are two clever strategies:
Click on a card to see the strategy! Both lead to the same answer: 59.
Telescope A discovers 46 new stars, telescope B discovers 28 more. This is where it gets trickier: the ones 6 + 8 = 14 – that’s more than 10!
A satellite has taken 58 photos, 23 of them are too blurry and are deleted. How many are left?
The first problem (58 − 23) leaves 35 photos.
A telescope picture shows 53 stars. 27 of them have already been marked with names. How many are still unmarked?
There are 88 officially recognised constellations in the sky.
The International Space Station takes about 90 minutes to orbit the Earth once.
Without addition and subtraction, astronomers couldn’t work out how many constellations are still missing or how long a journey takes.
When bridging through ten, people often forget that the tens place changes.
Example: 53 − 27 → ones: 3 − 7 doesn’t work → borrow → 13 − 7 = 6. Anyone who forgets that the tens place drops from 5 to 4 works out 5 − 2 = 3 instead of 4 − 2 = 2 – and gets 36 instead of 26.
Tip: always say the borrowing out loud or write it down on paper – then you won’t forget it!
Liri leafed through her star map. 59, 74, 35, 26 – numbers that would have seemed out of reach not long ago now stood among her memories.
From numbers up to 20 to numbers up to 100 – with clever strategies instead of fingers.