The rule is in the roof. It is the same for every floor.
On the left there is a number, on the right you write its result.
Example: the roof shows + 19.
The first floor has the 67. What goes next to it?
67 + 19 = 86, so the 86 belongs next to it.
Tip: go over the round ten. 67 + 20 = 87, and because you took one too many, take one away again: 86.
The number filled in shows the solution.
Example
For parents
What does this exercise practice?
Apply the same rule confidently to several numbers and calculate cleverly by going over the round ten.
Common pitfalls
Some children add up the numbers of the floors. Ask: what does the roof say, and what do you do with it to every number?
How to practice at home
Think up a rule ("always 9 more") and call out numbers to each other in turns. Whoever applies the rule correctly first thinks up the next one.
Related: Number House, Math Caterpillar, What does multiplying mean?