How long must $542 be invested at a rate of 7% to earn $303.52 in interest

The number of years that must be invested at  a rate of 7 % to earn$ 303.52 in interest is

    8 years

 calculation

                   P  is the principal   money to be invested =  $ 542

                       r= rate= 7/100=0.07

                      t=  time required

=$ 845.52=$ 542( 1+  0.07 t)

    =$  845.53 -$542  = $37.94 t

   =$303.52 =$37.94 t

  =  $303.52/ $   37.94  =  $37.94t/$37.94

  t=  8 years

c. because step up transformer steps up the input voltage i.e increases the input voltage  

the number of turns are greater on the secondary side of step up transformer  

transformer does not handle dc current