Consider hong kong (latitude= 22.3964° n, longitude= 114.1095° e, gmt+8), the value of direct solar irradiance on the 23rd of march at solar noon is 0.48 kw/m2 on an horizontal plane and the daily average solar irradiation on an inclined plane at latitude and south facing is 4.8 mj/m2. the maximum ambient temperature in march is typically around 22.5º c.

The interest rate would be 2.4%.

We start out finding the total amount he pays, including interest. 

78.40/month * 12 months * 5 years = $4704

We take off the amount of principal:
4704-4200 = 504

This is the amount of interest he pays.  Now we use the simple interest formula, I=prt:

504=4200(r)(5)

Multiply 4200 and 5:
504=21000r

Divide both sides by 21000:
504/21000 = 21000r/21000
0.024 = r
2.4% = r