Let A involves 30 units, B involves 15 units and C involves 20 units of work.
One welder does 3 units of A and 1 unit of C in a day.
One blacksmith does 1 unit of B and 2 units of C in a day.
Two welders would take 5 days to complete A. They finish A at the end of day 5.
On day 6 they finish 2 units of C.
Three blacksmiths finish B at the end of day 6.
After days 6, two welders and three blacksmiths together would take 18/8 days to finish the rest of C.
So in all it takes
days to complete all the jobs.