Suppose, the daughter nucleus in a nuclear decay is itself radioactive. Let `lambda_p and lambda_d` be the decay constants of the parent and the daughter nuclei. Also, let `N_p and N_d` be the number of parent and daughter nuclei at time `t`. Find the condition for which the number of daughter nuclei becomes constant.

Explanation

To find the condition under which the number of daughter nuclei becomes constant in a radioactive decay process where the daughter nucleus is also radioactive, we can follow these steps:

### Step 1: Understand the decay process
In a radioactive decay process, a parent nucleus (P) decays into a daughter nucleus (D). If the daughter nucleus is also radioactive, it will further decay into another nucleus (let's call it X).

### Step 2: Define the decay constants and number of nuclei
Let:
- \( \lambda_p \) = decay constant of the parent nucleus (P)
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