yo for this question, I'm unsure how to approach. We cannot just say that in 1200 years, x has 3 half lives and its daughter y has 4 half lives, therefore x is double y. This approach assumes that the initial amount of x and y is the same.
How should this question be approached?
By statistical law $$\frac{dx}{dt}=-λ_1x$$ Here $λ_1$ is the rate constant of decay of the sample X which can be found out from the half life. $$\int _X^x\:\frac{dx}{x}=-\int _0^t\:λ_1t$$ where $X$ is the initial amount of sample X. After solving the integral we get $$x=Xe^{-λ_1t}$$
Next, we approach Y. The amount of sample y=X-x=$X\left(1-e^{-λ_1t}\right)$. We write a similar rate law for sample Y too. $$\frac{dy}{dt}=-λ_2y+λ_1x$$ Here $λ_2$ is the rate constant of decay of the sample Y which can be again found from the half life. $$\frac{dy}{dt}=-λ_2y+λ_1Xe^{-λ_1t}$$ Solve this linear differential equation to get: $$y=\frac{λ_1X}{λ_2-λ_1}\left(e^{-λ_1t}-e^{-λ_2t}\right)$$ Now all that is left to do is substitute values, this is the approach.
