Question 56503
Let the number of lettuce has been sold as n


End of Monday, the lettuce would be 80-n


Double the leftovers = 2(80-n) = 160-2n 


End of tuesday (160-2n) - n = 160-3n


Triple the leftovers = 3(160-3n) = 480 - 9n



He sold the same number of head of lettuce that day too.  
However, at the end of that day there were no heads of lettuce left.


This means the 480-9n=n --> 480 = 10n --> n=48

* How many heads of lettuce were sold each day? --> 48
* How many heads of lettuce were left over at the end of each day?

Left overs on: 
Monday = 80-48 = 32
Tuesday = 32*2-48=16
Wednesday = 16*3 - 48 = 0