The Blue Ridge Tunnel,

The Blue Ridge Tunnel was designed by Claudius Crozet, and he was an engineer who served in Napoleon Bonaparte's army. It opened in 1858 and at that time it was the longest railroad tunnel in the world (at 4,273 ft.). The West Portal was 57 ft. higher than the East End. It was used until 1944, when a new tunnel was completed. The old tunnel is now known as the Crozet Tunnel.

If the workers drilled about 20 ft. a month and the Crozet Tunnel is about 4,280 ft. long, than how many months did it take to drill the entire tunnel?