Explanation
Heat lost by copper = heat gain by water,, i.e
[tex]\begin{gathered} -Q=+Q \\ -m_cc_c\mleft(T_3-T_1\mright)=m_wc_w\mleft(T_3-T_2\mright) \end{gathered}[/tex]
From the table,
For the first iron heated to 60°C
[tex]\begin{gathered} T_1=60,T_3=25 \\ -m_cc_c(25-60)=m_wc_w(25_{}-T_2) \\ -m_cc_c(-35)=m_wc_w(25_{}-T_2) \\ m_cc_c(35)=m_wc_w(25_{}-T_2)----i \end{gathered}[/tex]
For the second copper heated to 75°C
[tex]\begin{gathered} T_1=75,T_3=31 \\ -m_cc_c(31-75)=m_wc_w(31_{}-T_2) \\ -m_cc_c(-44)=m_wc_w(31_{}-T_2) \\ m_cc_c(44)=m_wc_w(31_{}-T_2)----ii \end{gathered}[/tex]
For the third copper heated to 90°C