1. The problem statement, all variables and given/known data i) The average concentration of 222Rn in air inside a building is 20Bqm-3; what is the average concentration of Rn in atoms per cubic meter? ii) In a sealed room of volume 50m3 what activity of 238U in a piece of porous rock would eventually give rise to this 222Rn concentration? Assume that all of the 222Rn produced in the rock escapes into the air. 2. Relevant equations R=λN N=N0exp[-λt] τ=1/λ 3. The attempt at a solution i) 20Bqm-3 exists in air. Since R=λN N=R/λ but: N = mass*NA/molar mass unknown mass, unknown decay constant... Different approach λ=ln(2)/half life unknown half life, no given mean life. Stuck. I'm sure I'm being stupid... I just don't see it. ii) Transition rate problem...not sure where to start from. Any equations I'm missing? Please help. Thanks in advance.