24, because every 5 feet the shadow is 2.5. so for the building's shadow the building is 10 feet for a 5 foot shadow. and 20 feet for a 10 foot shadow. plus the 2 shadow feet is 4 because its the shadow times 2 which is 2x2 which is 4. so you add 20 and 4 which is 24. if that makes sense.