Hello,

Let a=surface area of ice

t=thickness(1 cm)

Hence mass m=density*a*1=0.9*a*1

=0.9a grams

The heat required= Q=mL

=0.9A*80=72 calories

Mean thickness=

d=3+(4/2)=3.5 cm

Hence Q is given by-

Q=(K*a*(delta thetha))/d

t = Qd/K*a*(Delta thetha)

= 72A x 3.5 / 0.005 x A x 20

=2520 seconds.

Hence total time taken would be 2520 seconds or 2520/60=42 minutes

Hope this helps.