This applet allows you to see how two quantities co-vary according to an invariant relationship.
An altar in a church needs to be lighted, one special candle at a time continuously for a week-long festival. If the church uses special candles that last 7 hours each, then the church needs 24 such candles. If the church uses special candles that last x hours each, then the church needs y such candles. How is y related to x?