4, 20, 100, 500, 2,500, 12,500
what you do is 500 x 5 which equals 2,500 then times that by 5 and you get 1 12,500
the next two number in the pattern are 2,500 & 12,500
If we take the 68.73 to be the 100%, then what is 12 in percentage of that?
since in this case, the tip seems to be a percent of the bill
so

solve for "x"
5,1 -3,7 = 5,867
because when u do your calculation form 5,1 to-3,7 you get 5,867