The maximum throughput for Slotted aloha is $36.8$%, $0.368$
Hence, effective bandwidth $= 0.368*60kbps=22.08 kbps$
Bits per second outputted by each station $= 1024/40 = 25.6 bps = 25bps$
N station outputs 25 bits on a channel which has the effective B.W of 23.16 kbps
So,
$ (22.08*10^3)/25 = 22080/25= 883.2$, So 883 stations