Answer:
1.50
Explanation:
Let h and f represent the cost of 1 hamburger and 1 order of fries, respectively. Then you can write two equations for the cost of the orders.
... 5h +3f = 9.90
... 3h +5f = 8.50
Using Cramer's rule, the cost of a hamburger can be found to be ...
... h = (3·8.50 -5·9.90)/(3·3 -5·5) = (25.50 -49.50)/(9 -25)
... = -24/-16
... h = 1.50
The cost of one hamburger is 1.50.
_____
Formulation of Cramer's Rule
For equations ...
... ax +by = c
... dx +ey = f
The rule tells you ...
... x = (bf -ec)/(bd -ea)
... y = (cd -fa)/(bd -ea)
_____
Note that we have chosen this method of solution because we only needed the value of one of the variables. It is effectively the same as substitution (or elimination), but does all the steps in one formula.
Using the matrix solution methods on a calculator is another fast way to solve a pair of equations like this.