Please help: Noor and Hanif each poured 10cm3 of hydrochloric acid into a test tube and measured the temperature. Then they added identical pieces of magnesium ribbon. When the reaction stopped they measured the temperature again

noor's results
start temperature in Celsius: 18
end temperature in Celsius: 42

Hanif's results
start temperature in Celsius: 21
end temperature in Celsius: 45

the question: how did Noor and Hanif know when the reaction had finished?