Change in length of combined rod = Change in length of rod A+ Change in length of rod B So, ∆l‌=∆lA+∆lB ‌=lAαA∆T+lBαB∆T Here, ∆T=Tf−Ti=230−30=200∘C lA=lB=50‌cm=50×10−2m αA=2×10−5‌∘C−1 and αB=1×10−50‌∘C−1 So, ‌‌∆l=50×10−2×2×10−5×200 +50×10−2×1×10−5×200 =3×10−3m=3‌mm