Memory population T420

My T420 server has a single processor and 32 GB (4 × 8 GB – DIMM slot population A1, A2, A3, A4).

I'd like to populate all “A” slots, with 8 GB modules (6 × 8 GB – 48 GB).

But In the manual, it has only options with 6 × 16 GB and 6 × 32 GB.