Q $3.$ A flat$-$plate solar collector is subjected to a normal irradiance of $850\frac{W}{m^2}$ with an overall

heat loss coefficient of $5\frac{W}{m^{2}C}.$ The transmittance$-$absorptance product of the collector is $0\mathrm{.}8$

which makes the collector efficiency $60\%.$ If the ambient temperature is $10C$ and collector area

is $3m^2$ then find: $-$ a$)$ Useful heat gain. b$)$ Plate mean temperature. c$)$ Stagnation temperature.

Solve it this is from solar energy course