On the second day, he collected 3 times the number of tins he collected on the first day, which is 3*50 = <<3*50=150>>150 tins.
On the third day, he collected 50 tins fewer than the second day, which is 150-50 = <<150-50=100>>100 tins
The total for the three days is 150+100+50 = <<150+100+50=300>>300 tins.
To reach his goal, he still needs 500-300 = <<500-300=200>>200 tins.
Since the total number of days left in the week is 4, he'll need to collect 200/4 = <<200/4=50>>50 tins per day to reach his goal
#### 50