To solve this problem, let's calculate the workrate of each worker.
Let's assume that the total work required for each job is 1 unit.
Since the welder can finish job A in 10 days, their workrate is 1/10 units per day for job A. Similarly, the blacksmith's workrate for job B is 1/15 units per day.
Now, let's calculate the combined workrate for job C. Since the welder can finish job C in 20 days, their workrate is 1/20 units per day for job C. Similarly, the blacksmith's workrate for job C is 1/10 units per day.
On the first day, two welders begin work on job A, so the work completed on day 1 for job A is 2 * (1/10) = 1/5 units.
On the second day, three blacksmiths begin work on job B, so the work completed on day 2 for job B is 3 * (1/15) = 1/5 units.
Now, let's calculate the combined workrate for job C. Since the welder and blacksmith are working together, their combined workrate for job C is (1/20) + (1/10) = 3/20 units per day.
To find the least time required to complete all three jobs, we need to find the time it takes to complete the remaining work for job C, which is 1 - (1/5 + 1/5) = 3/5 units.
To find the time required to complete the remaining work for job C, we can divide the remaining work by the combined workrate for job C: (3/5) / (3/20) = (3/5) * (20/3) = 4 days.
Therefore, the least time required to complete all three jobs is 4 days.
The correct answer is A) 33/4 days.