b=0.7 * 34 = 23.8
a=1.2 * 23.8 = 28.56
The first team (A and C) do the work like so:
1/28.56 + 1/20
the second team (B) does the work like so:
[To understand the foregoing, consider the following:
party Q does 1/2 the work in one day
it takes party Q 2 days to do 2/2, or all, of the work.]
So, day one, again looks like the following:
0.085 of the work gets done
day two looks like the following:
0.042 of the work gets done
adding the two days together, we see that in two days, 0.127 of the work gets done.
So divide 1/0.127, and we see the number of pairs of days it takes to get the work done.
That number is 7.874
Multiply that figure by two to see the number of total days on the job:
It is easier, perhaps, to look at the fractions, or the decimals in terms of percentages.
For example, if 8.5% of the work gets done in day 1, and in day 2, 4.2% of the work gets done, together, they are about 12.7% of the work done each two days.
divide 100% of the work by the two-day percentage of 12.7% and you get 7.874, which needs to be multiplied by two, since the question asks for days and not pairs-of-days, so it’s, again, 15.75 days.