On the second day, they collected 20 + 5 = <<20+5=25>>25 cans.
On the third day, they collected 25 + 5 = <<25+5=30>>30 cans.
On the fourth day, they collected 30 + 5 = <<30+5=35>>35 cans.
On the fifth day, they collected 35 + 5 = <<35+5=40>>40 cans.
Thus, the goal of William's class is to collect a total of 20 + 25 + 30 + 35 + 40 =<<20+25+30+35+40=150>>150 cans of food per week.
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