An employee of a company works on the 15th floor of an office block. When she arrives at work she takes the lift, usually with other people, as most employees arrive at work at a similar time. When this happens she travels in the lift to the 15th floor.
Occasionally she arrives late using flexi-time, and often has the lift to herself. On these occasions she travels to the 13th floor and then walks up the stairs.
When leaving work, at the normal time or not, she travels in the lift to the ground floor, and never walks at all.
She doesn't like walking, so why does she do so when arriving late?
Answer at 9.00 on Monday