At first, (typed in black, blue pens = 25% (u) and red pens = 75%(3u) )

Blue     [u][u]      <—– (Bought an equal number of blue pens means another u added)

Red      [u][u][u]

So new total pens = 5u

New blue pens = 2u

Hence blue pens in the end = (2u)/(5u) x 100% = 40%