A student uses 600 clay models of caterpillars to investigate how camouflage affects survival against bird predation.
She paints 300 of the clay models green to match the leaves of a hedge, and 300 of the models yellow so they are highly visible.
She attaches the models to the leaves in the morning, and checks them the next day for signs of bird attacks (peck marks).
The bar graph shows her results:
Calculate the difference between the number of damaged yellow model caterpillars and the number of damaged green model caterpillars.