Related Clues:

• The only even prime number
• A quarter of eight?
• Box-office bonanza
• I and I
• Duet necessity
• Chartbuster
• Tandem bike's capacity
• Four halves
• Helium's atomic number
• Partner of 40-Down, traffic-wise
• 40 Across and 40 Across
• Successful turn in Battleship
• Company quota
• ___ the ground running
• Timing lead-in