The activity of a sample of caesium-137 is initially 40 kBq. How long will it take for the activity of the sample to fall by 30 kBq? (The half-life of caesium is approximately 30 years.)

The ideal gas model is valid if which of the following conditions is true? the gas density is low. the gas density is high. the temperature is low. the temperature is high. the gas density and the temperature are low. the gas density and the temperature are high. the gas density is high and the temperature is low. the gas density is low and the temperature is high.