i don't know why (theta)L/(theta)o = 0.9

please help

4-59 A cube of aluminum 10cm on each side is initially at a temperature of 300C and is immersed in a fluid at 100C. The heat-transfer coefficient is 900W/m2C. Calculate the temperature at the center of one face after 1min. L=5cmTi=300CT=100Ch=900m2CW=60seck=204mCW=8.42105m2/shLk=4.53L2=2.02i0=0.70L=0.9Centerofface:i=(0.7)(0.9)=0.63T=(0.63)(300100)+100=226C Figure 4-7 I Midplane temperature for an infinite plate of thickness 2L:(a) full scale