Have a look at the attached sample.
basically it needs a unstored calculated repetition field (20 repetitions). If you use a number field for the InUse field (1 in use, empty or 0 not in use) and the car IDs are from 1 to 20, then the formula could be this:
ExecuteSQL ( "
" ; "" ; "" ; Get ( CalculationRepetitionNumber )
Then show the repeting field on the layout in 4 occurrings: 1-5, 6-10, 11-15, 16-20, set the conditional formatting to: if value=1 set background color...
CarsInUse.fmp12.zip 67.1 K
Thanks Jason and Otmar. That attachment was perfect Otmar.. thank you!