the entropy change for a reversible adiabatic process is zero as it remains constant. Is this a reversible process?
assuming T1>T2:
hot (h) water has mass M, temp T1
cold (c) water has mass nM, temp T2
let the final temperature be Tf
if δQ=0 as the process is adiabatic, |Qh|=|Qc| so Qh=-Qc...