Divide 2000 over 67 to get 2000/67 = 29.85 approximately
Round down to the nearest whole number to get 29
Note how 29*67 = 1943 which is less than 2000 while 30*67 = 2010 which is over 2000
Subtract 1943 from 2000 to get 2000 - 1943 = 57. It's unlikely that Tonka is 57 pounds or less. So that means Tonka will have to leave out one box (or more). It's not given how much Tonka weighs so we can't get the exact answer here.
If Tonka has x = 28 boxes, then 2000 - 67*x = 2000 - 67*28 = 124 pounds are left for Tonka. If Tonka weighs this much then this would work, though again the teacher doesn't say what the person's weight is.