Question 55075
You are going to invest $25,000.00, part at 14% and part at 16%.  What is the most that can be invested at 14% to make at least $3,600.00 interest per year? 

ans:  let x be the amount invested at 14%
      and y be the amount invested at 16%

hence total amount = x+y = 25,000.00    -------- I

      total interest according to the problem will be
                   .14*x+.16*y=3,600.00  --------- II

then value of x to be found out by solving above two equations.
     Multiply eq. I by .16 and subtracting from II Will give

                 (.14*x+.16*y)-.16*(x+y)=3600-.16*25000
                  .14x-.16x=3600-4000=400
                  .02x = 400
                     x = 400/.02 =20,000
       hence invested at 14% = $20,000