Yeah, in the equation 5+50*20. But in the way the post phrases the order, it is, as someone else commented, (20(5s+50)+1015)-y=100s+a, where s is shoe size, y is year of birth, and a is age.
This kind of things usually are made for a certain year, it probably told you the age you were last year? I did it last year and it worked and now it's an year off
1005+1015=2015
I think that has something to do with it
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Wait never mind, it worked this time.