Learn basic math on line

1. Problem:  The sum of twice a number plus 13 is 75.  Find
             the number.
   Solution:
             The word is means equals.  The word
             and means plus.  Therefore, you can
             rewrite the problem like the following:
             The sum of twice a number and 13 equals 75.
      

             Using numbers and a variable that represents 
             something, N in this case (for number),
             you can write an equation that means the same thing
             as the original problem.
      

             2N + 13 = 75
      

             Solve this equation by isolating the variable.
      

             2N + 13 =  75     Equation.
                - 13 = -13     Add (-13) to both sides.
             -------------
             2N      =  62
             N       =  31     Divided both sides by 2.
      

2. Problem:  Find a number which decreased by 18 is 5 times its
             opposite.
   Solution:
             Again, you look for words that describe equal quantities.
             Is means equals, and decreased by
             means minus.  Also, opposite always means
             negative.  Keeping that information in mind makes it
             so an equation can be written that describes
             the problem, just like the following:
      

             N - 18 = 5(-N)        Equation.
               
             N - 18 = -5N          Multiplied out.
            5N + 18    5N + 18     Add (5N + 18) to
            ------------------     both sides.
            6N      =       18
                  N = 3            Divide both sides
                                   by 6 to isolate N.