Endless possibilities in exploring addition, subtraction, counting on and general counting up to 20. How may windows do you see? subtract the number of houses without chimneys from the total with. How many houses have 4 windows. Made from a tough semi-rigid plastic, these new overlays are long lasting and self-supporting, so that they can be propped up in front of any whiteboard or big book easel. They can also be hand-held or fastened to a wall. Compatible with the majority of whiteboard markers. Size: 720 x 635mm Please Note: These Overlays should be stored Flat Also available an Easel (code 1a) and Table Top Whiteboard (code 300), with rear storage area for the overlays.