A long steel rod 1 in. in diameter is initially at a uniform temperature of 1.200°F. It is suddenly immersed in a quenching bath of oil at 150°F. In 4 min its average temperature drops to 250°F. How long would it take to lower the temperature from 1,200 to 250°F 

(a) If the rod were 2¹/₂ in. in diameter. 

(b) If it were 5 in. in diameter? For steel. k=26 Btu/ft-h-°F; ρ = 486 lb/ft³: C_p = 0.11 Btu/lb-°F.