Bacteria that have been exposed to two different bacterial strains: one carries only a gene for resistance to the antibiotic Ampicillin; the other carries the gene for Ampicillin resistance as well as a gene for green fluorescent protein (GFP), which glows under UV light. Some bacteria took up the Ampicillin-resistance gene, others took up the genes for Ampicillin resistance and for GFP, but some took up neither (non-transformed). The bacteria were then spread on petri dishes with either nutrients and Ampicillin or just nutrients.

Why do you plate only a small amount of the bacterial solution on the Ampicillin-treated plates, and an even smaller amount on the plates without Ampicillin, when you are trying to calculate transformation efficiency.