A CERTAIN NUMBER OF SPECIAL SPELLS MUST BE CAST TO PROTECT THE SECR...

7. A certain number of special spells must be cast to protect the secret hiding place of the Order of the Phoenix. Each member of the Order is capable of casting the same number of these spells per hour. If two wizards were to join the Order, casting the protection spells can be completed in three hours instead of four hours. How many wizards are in the Order?