If there are 40 people, each person gets 600/40 = <<600/40=15>>15 cans.
Thirty percent of the total number of people present is 30/100*40 = <<30/100*40=12>>12
If there are 30% fewer people, the number of people who are to be fed is 40-12 = <<40-12=28>>28
Since each person is given 15 cans, twenty-eight people will need 15*28 = <<15*28=420>>420 cans.
#### 420