If there's a 40, a 24, and a 10, the 40 is the READ and 24 is the WRITE.
The 24x write is probably 3-zone CLV (if I remember right)...
Starts at 16x, then zones-up to 20x and then to 24x as rotational speed allows.
One thing's for certain - a 48x writer is NOT twice as fast as a 24x.
Rotation speed means the 48x will start a llittle faster (maybe 18x-20x), and may gain further by using CAV mode instead of zoned (constant rotation, data rate continually increasing).
But it will NOT be twice as fast - for short burns, only the starting speed difference will matter
So if the price is right - and if faster drives are harder to find, and grossly overpriced, it's probably not a bad drive to have.
A 24x drive can also be a little more forgiving on media - since some 40/48x media really isn't up to the task at full speed.