Five classes in the sixth grade donated 6300 yuan, of which one class donated 1400 yuan, and the second class donated less than the first class 100 yuan. The donations of Class 3 account for 20% of the total number of grades, and Class 4 is 6/7 of Class 5.
∴ 1400+ 1400- 100+a+6/7*a=6300( 1-20%)
13/7*a=5040-2700
a=2340*7/ 13= 1260
∴6/7*a=6/7* 1260= 1080
Answer; Four classes of donations 1080 yuan.