When sandy and allen's mother decorates the house by herself, it takes her 5 hours. When both Sandy and Allen help, the three can finish the job in 2.5 hours.

If you're asking how this is possible,

Answer and explanation:

If Sandy and Allen's mother does the job in 5 hours

And Allen and Sandy's contributions completes the job in 2.5 hours

5 hours - 2.5 hours =2.5 hours

Allen and Sandy's contribution is half of the time it took only Allen and Sandy's mother= 2.5 hours

Therefore Allen and Sandy together will take 2.5×2 = 5 hours to complete the job alone without their mother

A similar example: if it takes one person 2 hours to peel 5 potatoes and it takes 1 hour to peel same number of potatoes if a second person helps, then this second person reduces the time used by 1 hour(50%) and would take 2 hours to complete the work.