I was taught a long time ago this really useful trick to calculate an unknown value when you know 3 existing values that are related. An example would be something like:
If there are 21 peas in 3 pods how many are there in 17 pods?
This gives us 2 values that have a relation, the next value that fits a certain rule that would reveal the missing value.
I was taught to write it down in a kind of 3 out of 4 square, and you would multiply diagonally and divide up and down. But I can't quite remember the exact rule and if this little trick has a name.