Answer:
Henry = 12 hours
Robert = 10 hours
that 2 people
so 12/ divided by how many people
and 10/ divided by how many people
so 12/2
and 10/2
= 5 and 6
then find the (median) middle of both numbers one or more
which is 5 1/2
so they can finish working together in 5 1/2 hours