Ok. first, make an equation for volume:
X mL of coke +Y mL of Sprite = 150mL
X ml of coke = Y ml of Sprite - 150 ml
or more basically:

X = Y - 150

Now make an equation for cost:
((0.02 $/mL) * X mL of coke) + ((0.05 $/mL) * Y mL of Sprite) = $4.32

review your units to be sure they make sense - so the volume of each soda cancels out the cost per volume and then you add up each cost for the total. Check.

So now let's create a simpler equation without the units:
0.02X + 0.05Y = 4.32

Now combine the equations so that you only have one variable. Since we solved for X (XmL of Coke =150ml - YmL of Sprite) - let's substitute the Y side of the volume equation into the X of the cost equation:

0.02 (150 - Y) + 0.05 Y = 4.32

Now solve for Y - which will be in mL, but we'll continue to leave the units for simpler presentation (since I already checked unit math above):

3 - 0.02Y + 0.05Y = 4.32

0.03Y = 1.32

Y = 44

So there were 44 mL of Sprite in the mix.

With that, we can go back to the first equation and solve for X. You could use either equation, but the first one is simplest:

X = 150 - Y

X = 150 - 44

X = 106

So there were 106 mL of Coke.

Double check your work by plugging these numbers into the second equation:

0.02X + 0.05Y = 4.32

0.02 (106) + 0.05 (44) = 4.32

2.12 + 2.2 = 4.32

4.32 = 4.32 - CHECK!

This is a simulntaneous equations question. You can set up two equations from the text, and then solve them.

so, he has bought some volume c (in ml) of coke, and some volume s of sprite [use x and y if that is what you have practiced]. Our goal is to find out what s and c are.

Each ml of coke costs $0.02, so the cost of the coke he bought is $0.02/ml x c ml.
Similarly the cost of spite he bought is $0.05/ml x s ml

Add these together to get the total cost of drinks:
0.02c + 0.05s

We know that he paid $4.32, so, using the expression above:
0.02c + 0.05s = 4.32 (equation 1)

Just using this equation, there are two unknown variables, so there are lots of ways he could pay exactly this much. He could buy just coke, or a different volume of just sprite, or a variety of mixtures. But these mixtures would all have different total volumes, and since the question specifies he bought 150ml we can calculate the amounts of s and c he bought.

Remember s and c are the volumes of drinks he bought.
s+c = 150 (equation 2)

This can be substituted back into equation 1.
Rearrange: c = 150 - s
substitute c in equation 1: 0.02(150-s) + 0.05s = 4.32

See if you can go from there.

Hints:
Remember the goal is to find s and c....
solve for s
find c by substituting the value of s into equation 2

thnx guys

also john had a total of $4.65 in his pocket and i found out that i did it right but i misplaced a the decimal, the equation was: $0.05s+$0.02(150-s)=$4.65

we had to use only one equation, but i'm still learning it this year since i'm in the "IB program".

At a concession stand, the first cup of iced tea costs $2.25, and refills cost $0.15 each. If one customer ordered a cup of iced tea and 2 refills, how much did the customer pay?

Hi myles.todd ,
You would solve the equation like this
   2.25
+ 0.15
   0.15
  ______

Or you can multiply 0.15 x 2=N

Then 2.25 + N = Total of what customer payed 

Notice the word problem said " How Much did the customer pay?"
This means to use addition to find the Total the customer payed.