SOLUTION: please help with equation for interest if a principle of 2000 at rate of 6% earns 300, how much will principle of 4000 earn at 8%?

    +-----------------------------------------------------------------+
    |   If a principal of 2000 at simple annual interest rate of 6%   |
    |    earns 300 in some time interval, how much will a principal   |
    |          of 4000 earn at 8% simple annual interest rate         |
    |                 during the same time interval ?                 |
    +-----------------------------------------------------------------+

Use the formula for simple interest 

    I = P*r*t,    (1)

P is the principal, t is the time (in years), r is the annual simple interest rate,
I is the interest amount.


For the first scenario, we have

    300 = 2000*0.06*t.


From this equation, find time t =  = 2.5 years.


So, the time period is 2.5 years.


Apply the formula (1) AGAIN, this time for the second scenario

    I = 4000*0.08*2.5 = 800.


ANSWER.  The second interest amount is 800.