I need a refresher on calculus need to derive this equation, the question is: A --> B --> C and where decay constants: Derive an expression for the activity of B as a function of time. So what I'm having trouble with is changing the derivation for the number of N when lambda_A = lambda_B so that I derive the activity of B. Normally we would have: But because of the condition, we have to go about it differently. So setting the decay constants equal to each other: And this is where I get a little stumped trying to change the derivation. When lambda_A doesn't equal lambda_B, the derivation ends up with: The problem I'm having is that I'm not sure what has to be done differently in the original derivation to solve for activity, my calculus skills are a little rusty so if anyone has any suggestions or advice, it's greatly appreciated.