As steel scale is correct at 0℃. Actual length measured by steel scale at 30℃. L=100(1+t\ imes \\alpha )\\\\ L=100(1+12\ imes { 10 }^{ -6 }\ imes 30)\\\\ L=100(1.00036)cm\\\\ L=100.036cm Brass rod has length 100.036cm at 30℃ let's assume length L' is length at 0℃so - 100.036=L'(1+t\ imes \\alpha )\\\\ 100.036=L'(1+19\ imes { 10 }^{ -6 }\ imes 30)\\\\ 100.036=L'(1.00057)\\\\ L'=99.979cm