There are some interesting interpretations in this string.
But I think that we need to RTFQ.
The question did not ask how long it would take each individual to complete the task when working alone.
It asked "How long are A B C working?"
Nothing in the question suggests that any individual works less than the full specified time when part of a specified pair.
A works with B for 48 minutes and with C for 80 minutes. So A works for a total of 48 + 80 = 128 minutes.
B works with A for 48 minutes and with C for 60 minutes. So B works for 48 + 60 = 108 minutes.
C works with B for 60 minutes and with A for 80 minutes. So C works for 60 + 80 = 140 minutes.
"Words failurise me" ................................. George Dubbya Bush.
"Don't let them failurise you"................... RTFQ.