Question 963885
With a given perimeter, a square always has the most area, 
so all sides should be equal:
100cm/4=25 cm
She should order trays 25 cm by 25 cm. 
TABLE:
Length  Width  Pieces
-(cm)----(cm)-----#
---5------45------9
--10------40-----16
--15------35-----21
--20------30-----24
--25------25-----25
--30------20-----24
--35------15-----21
--40------10-----16
--45-------5------9