Answer:
t = 2244.3 sec
Step-by-step explanation:
calculate the thermal diffusivity


Temperature at 28 mm distance after t time = = 50 degree C
we know that

![( 50 -25)/(300-25) = erf [\frac{28* 10^(-3)}{2\sqrt{1.34* 10^(-5)* t}}]](https://img.qammunity.org/2020/formulas/engineering/college/61h0mchbt0s6yescjgk7ei8uj530n09wgt.png)

from gaussian error function table , similarity variable w calculated as
erf w = 0.909
it is lie between erf w = 0.9008 and erf w = 0.11246 so by interpolation we have
w = 0.08073
![erf 0.08073 = erf[(3.8245)/(√(t))]](https://img.qammunity.org/2020/formulas/engineering/college/n5p9bl2hmhq3tmte1z9dm5oft0kaggp4mh.png)

solving fot t we get
t = 2244.3 sec