here is it....first of all whats the formula for finding SI (simple interest)?
it is $$\frac{p*r*t}{100}$$
so in this we will place everything int heir place and solve!
now SI= Amount - Principal
so SI = 688 - 500
therefore SI =188
now lets place everything
Lets take rate to be 'r'
$$188=\frac{500*r*4}{100}$$
now divide or cut 500 by 100 and multiply by 4
so that gives us
$$188=20r$$
now divide 20 by 188 to get r
$$\frac{188}{20}=r$$
divde!
$$\frac{47}{5}=r$$
$$9.5=r$$
so the rate is 9.5%
to check the answer you can put 9.5 in rate's place and get 188...its correct!
any doubt?