Analysis: (12+3 1)×20÷2=430

"Olympiad Mathematics" is the abbreviation of Olympic Mathematics Competition. 1934- 1935, the former Soviet union began to hold middle school mathematics competitions in Leningrad and Moscow, and held the first international mathematical Olympics in Bucharest from 65438 to 0959 in the name of mathematical Olympics.

Arranging several numbers in a row is called a series, and each number in the series is called an item, in which the first item is called the first item and the last item is called the last item. The difference between the last term and the previous term of a series is equal to arithmetic progression, and the difference between the last term and the previous term is called tolerance series.