If Karen takes 2.5 hours to paint the whole wall, then in an hour she paints a fraction of 2/5.
On the other hand, if Stephanie takes 1.25 hours then in an hour she paints a fraction of 4/5 of the wall.
When the two are working together then one hour they will paint a fraction of 6/5 of the wall. But the wall is one whole therefore it will take 5/6 hours or 50 minutes for them to paint the whole wall together. The answer is 50 minutes