Looking at the numbers,
12 ---> 18
21 ---> 27
32 ---> 38
45 ---> 51
we can see that the difference between the numbers in each pair is 6. We need to add 6 to the number on the left to get the number on the right. We can use a spreadsheet to do the computations and test our rule.
Put any of the numbers from the left side of an arrow into cell A1 and the formula =A1+6 into cell B1. The computer will compute the result of using the rule. If it isnŐt obvious what the rule is, you may use a spreadsheet to experiment. You could try out a rule and use the spreadsheet to test it.
Note: This is an example of a function. For any number you input, the rule determines exactly one output.
Make up some rules of your own for your friends to guess.