The answer is 80.
The 1st 20 elements till Ca does not have 3d orbital filled. Ca valence electrons enter 4s orbital . Filling of the 3d orbital begins after 4s orbital .So the Sc is the first element to have electrons in 3d. After that all elements till 100 has electrons in 3d orbital. so from 21 to 100 all have 3d electrons.