= 1999×(2000- 1998)+ 1997×( 1999- 1996)+ 1995×( 1996- 1994)
= 1999×2+ 1997×2+ 1995×2
=2×( 1999+ 1997+ 1995)
=2×599 1
= 1 1982
2. According to the meaning of the question, orchestras are divided into three categories, namely, students in grades five and six and beyond (the number of students in grades six and five and six is 170, and the number of students in grades five and six is 135. 170+ 135 can get students in grades 5 and 6 and people twice as many as those outside grades 5 and 6)
( 170+ 135-205)/2
= 100/2
=50 (person)
3. First get the speed of B, 700/5-60= 140-60=80 (m), then the speed of A is 60+ 10=70 (m).
According to the meaning of the question, the problem of B chasing A after B meets C is expressed as follows.
80/(80-70)
=80/ 10
=8 minutes
(Comprehensive type: 80/(700/5-60-60- 10))