what rate of simple interest was $500 invested at, if it accumulated to $688 in 4 years?

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?