Question 60861
Let the cost of the chocolate bar  = “x” 
The cost of the pack of the gum = “y”

Sally bought 3 chocolate bars and a pack of gum = $1.75
3(chocolate bars) +(pack of a gum)= $1.75
3x + y = $1.75                     (1)

Jake bought two chocolate bars and 4 packs of gum =$2.00
2(chocolate bars) + 4 ( packs of a gum) = $2.00
2x + 4y = 2.00                    (2)


Solving the simultaneous equation, we get
3x + y   = 1. 75
2x  + 4y = 2.00

Now multiply equation(1) by 4,
3x * 4  + y * 4   = 1. 75 * 4
2x  + 4y = 2.00 

We get,

12x + 4 y =  7        (3)
 2x +  4y  =  2        (2)

Subtract the two equations, we get

10x  = 5
x  = (5/10)
x = (1/2) = 0 .5

Now we shall find the value of “y” by substituting the value of “x” in  equation(2), we get
2(1/2) + 4 y = 2
1 + 4y = 2
4y = 2 -1
4y = 1
y = (1/4) = 0.25

Therefore the value of “x” is 0.5 and value of y is  0.25.
That is  
The cost of chocolate bar   = $0.5
The cost of the pack of gum = $0.25