Answer:
50 black keys
Step-by-step explanation:
for every set of 7 white keys there are 5 black keys
72/7 = 10
so there is a total of 10 full sets of keys
then you have 2 white keys remaining but that doesn't matter because they will just be on the dies and already have a black key next
so this means that you only have to calculate the number of 10 sets of black keys and thats 5 x 10.
hope this helps : )
Set it up as a ratio. $5.25/5 = $1.05 per notebook
Then solve your equation by multiplying the cost by 3. 3 * $1.05 = $3.15
Keep in mind that whenever you have a variable inside an expression, you have to think of it as a placeholder, hiding a certain value which we haven't specified yet.
Expressions with variables are a mean to express the generic idea underneath the expression, rather than its particular value.
So, an expression like x-7 represents the idea of subtracting 7 from a particular number. Once you give a specific value to x, the expression will become an actual subtraction between numbers, and you will be able to compute its value.
So, we have several values for x, which are several ways to turn our "abstract" subtraction into an actual, computable subtraction.
If 
, the expression becomes 
and it evaluates to 
.
If 
, the expression becomes 
and it evaluates to 
.
If 
, the expression becomes 
and it evaluates to 
.
If 
, the expression becomes 
and it evaluates to 
.
If 
, the expression becomes 
and it evaluates to 
.
She would get £2.50
4 times 2 is 8
2 times 3 is 6
0.7 times 5 is 3.5
Add them together
Minus it from 20
It equals £2.50
Mrs. Blat spends at least ...
13.99 +10.00 +6.00 = 29.99
each month. Over the year, this is a minimum of
12*$29.99 = $359.88
_____
The variable cost is the cost of the 2nd book, which may or may not be purchased in any given month.
