Find number abc

There is a story about a monastery perched high on a cliff several hundred feet in the air. The only way to reach the monastery was to be suspended in a basket which was pulled to the top by several monks who pulled and tugged with all their strength. Obviously the ride up the steep cliff in that basket was terrifying. One tourist got exceedingly nervous about half-way up as he noticed that the rope by which he was suspended was old and frayed. With trembling voice, he asked the monk who was riding with him in the basket how often they changed the rope.
The monk thought for a moment and answered brusquely, "Whenever it breaks."