Answer
95625 houses
Work Step by Step
We have
$\eta = \frac{Q_1- Q_2}{Q_1}$
Dividing numerator and denominator on right side by $t$ we get
$\eta = \frac{P_1 -P_2}{P_1}$
$0.32 = \frac{900MW } { P1}$
$ P_1 = 2812.5 MW$
Now $ P_1 -P-2 = 900MW$
$ 2812.5MW - P_2 = 900Mw $
$P_2 = 1912.5MW$
Power used by house is $20kW$. Therefore total number of houses which could be heated from waste heat is
$ n= 1912.5MW/20kW$
$n= 95625 houses$
