Question 10342
  First of all form equations.

  Let the cost of 1 doughnut be = x
  Let the cost of 1 large coffee = y

So  3x + 4y = $4.91    ........... (1) Monday's purchases
    5x + 6y = $7.59    ........... (2) Tuesday's purchase
Eq.(2)* 2, we get
    10x + 12y = $15.18 ........... (3) 
Eq.(1)* 3, we get                       
    9x + 12y = $14.73  ........... (4) 
Eq.(3)-(4), we get
    x = $0.45
Now replacing the value of x in Eq.(1), we get
    (3*0.45) + 4y = $4.91
=    $1.35 + 4y = $4.91
=    4y = $4.91 - $1.35 = $3.56
So   y = {{{3.56/4}}} = $0.89

Hence x = $0.45 and y = $0.89
i.e. cost of large coffee = $0.89  Answer


First form the equations.  Then try to eliminate one variable either by multi-
plying or dividing the equation so that at least one variable becomes equal to 
the same variable on the 2nd equation.  Then you can eliminate that variable 
either by adding or subtracting as the case may be. i.e. if the signs are different, then add and if the signs are same, then subtract to remove the particular variable.  Then there will be an equation with only one variable which can be solved easily.  

I hope this is very clear to you.  Try to solve some problems on your own. OK
Best of luck.  God bless you.


gsm