Where a is the (constant) processing time per person, b is the time per item, x1 is the number of people in line 1, y1 is the number of items in line 1, x2 is the number of people in line 2, and y2 is the items of people in line 2.
a(x1) + b(y1) < a(x2) + b(y2) a(4) + b(11) < a(1) + b(y2) a(4) + b(11) - a(1) = b(y2) a(3) + b(11) = b(y2) 3(a/b) + 11 = y2 3(a/b) = y2 - 11
That is, line 2 must have 12 more items in it than three times the processing time per person divided by the processing time per item.