When the first bucket at the slow spring has X ltrs in it move it to the fast
spring and start filling the other backet at the slow spring. Then as both
springs are fully occupied the transit time does not add to the total fill time.
Choose X so both springs finish filling buckets at the same time, so:
[3x8+(8-X)]/2 = 8+X
X = 16/3,
and the time taken is:
t = 8 + 16/3 = 13 1/3 min.
Which can be cross checked, as both springs are fully occupied
during the filling process, so the total rate of filling is 3 ltrs/min
and there are 40 ltrs to fill so time = 40/3 = 13 1/3 as required.
Also all three people are required as we need two fillers and one
mover simultaneously while the part filled bucket is moved from
one spring to the other.