An exam board has admitted making a mistake in a physics A-level exam sat by students at around 1,200 schools and colleges.
Four schools have contacted AQA to complain after the paper, which students sat yesterday, contained a typing error.
The exam board has admitted a mistake was made but said the question could still be answered.
An AQA spokesman said: "We meant to ask students to compare a 150 kΩ resistor - not a 150 Ω resistor as it said in the paper - to a 300 kΩ resistor.
"At this stage it looks as though most students haven't been affected by this, but we're letting our examiners know and will make sure it doesn't affect students' grades."
Jack Leyland, who sat the exam, said the mistake was annoying, especially in a paper which had "awkward twists on usually familiar questions, which made it difficult".