This problem involves calculating the work rates of skilled and semi-skilled workers to determine how many semi-skilled workers are needed to replace skilled workers and maintain the task completion timeline.

1. Calculate Worker Rates

First, let's establish the total amount of work and the relative rates of skilled and semi-skilled workers.

• Let the total work required be $W$.
• 18 skilled workers complete the task in 12 days. The total work can be represented as $W = 18 \text{ skilled workers} \times 12 \text{ days} = 216$ units (where a unit is the work done by one skilled worker in one day).
• Let $R_S$ be the rate of one skilled worker. We set $R_S = 1$ unit/day.
• 24 semi-skilled workers complete the same task in 15 days. So, $W = 24 \text{ semi-skilled workers} \times 15 \text{ days} = 360$ units (where a unit is the work done by one semi-skilled worker in one day).
• Let $R_{SS}$ be the rate of one semi-skilled worker.
• To compare rates, we equate the total work: $216 \times R_S = 360 \times R_{SS}$.
• Finding the ratio of their rates: $ \frac{R_S}{R_{SS}} = \frac{360}{216} = \frac{5}{3} $.
• This means one skilled worker works $5/3$ times faster than one semi-skilled worker.
• If we use the skilled worker rate ($R_S = 1$ unit/day), the rate of a semi-skilled worker is $R_{SS} = \frac{3}{5} R_S = \frac{3}{5}$ units/day.

2. Determine Stipulated Time and Required Rate

The question implies maintaining a specific completion time. Let's determine this time based on the initial workforce mentioned.

• 12 skilled workers started the task.
• If these 12 skilled workers continued without interruption, the time to complete the total work (216 units) would be: $T = \frac{216 \text{ units}}{12 \text{ workers} \times 1 \text{ unit/day/worker}} = 18$ days.
• This 18-day duration is the likely "stipulated time".
• To complete the task in 18 days, the required average work rate is: $\text{Required Rate} = \frac{216 \text{ units}}{18 \text{ days}} = 12$ units/day.

3. Calculate Rate After Workforce Change

Analyze the workforce composition and rate after some workers leave.

• The initial group had 12 skilled workers, whose combined rate was $12 \times R_S = 12 \times 1 = 12$ units/day. This matches the required rate.
• 3 skilled workers leave, leaving $12 - 3 = 9$ skilled workers.
• The rate of these 9 skilled workers is $9 \times R_S = 9 \times 1 = 9$ units/day.

4. Calculate Required Semi-Skilled Workers

Determine how many semi-skilled workers are needed to bring the work rate back to the required level.

• Let 'x' be the number of semi-skilled workers needed to replace the 3 skilled workers.
• The new workforce consists of 9 skilled workers and 'x' semi-skilled workers.
• The new combined rate is $(9 \times R_S) + (x \times R_{SS})$.
• Substituting the rates: $\text{New Rate} = (9 \times 1) + (x \times \frac{3}{5}) = 9 + \frac{3x}{5}$ units/day.
• To meet the stipulated time (18 days), this new rate must equal the required rate of 12 units/day.
• Set up the equation: $9 + \frac{3x}{5} = 12$.
• Solve for x: $ \frac{3x}{5} = 12 - 9 $ $ \frac{3x}{5} = 3 $ $ 3x = 15 $ $ x = \frac{15}{3} $ $ x = 5 $

Therefore, 5 semi-skilled workers should replace the 3 skilled workers.