The profits are divided based on the product of the amount invested and the time for which the investment was made.
Let \( t \) be the number of months A worked.
A’s investment is ₹ 42,000 for 12 months. So, A’s share of the profit is proportional to \( 42,000 \times 12 \).
When B joins, the amount invested by B is ₹ 72,000. B’s share is proportional to \( 72,000 \times t \).
For equal profit distribution, the proportion of investments for both must be equal:
\[
42,000 \times 12 = 72,000 \times t
\]
Solving for \( t \):
\[
t = \frac{42,000 \times 12}{72,000} = 5 \text{ months}.
\]
Thus, B joined after 5 months.