Which sentence is more formal?

 - Andrew Carnegie made tons of dough when he sold his steel company to John Pierpont Morgan in 1901.
 - Andrew Carnegie became one of the world's richest men after selling his steel company to John Pierpont Morgan in 1901.
The second sentence is less formal. You can tell because it uses slang (tons of dough).
The first sentence uses formal language in place of the slang, so it is more formal overall.