Keywords
linear equation, variables
Step 1
Define the variables
Let
x-----> number of erasers
y-----> total cost
we know that
------> this is the linear equation that represent the situation
therefore
the answer is y = 0.84 + (0.38)x
For 175 : 250 divide them both by 25 to get

<

25 goes into 175 > 7 times > 25 into 250 is 10 times! >

For the second one 16 : 40 just divide the top and bottom by 8 since 8 is a common factor of both which will give us

.
8 goes into 16, 2 times and 40, 5 times.
This is a sum and difference problem, with the equations
R-N=35
R+N=635
The solution of which is
R=(635+35)/2=335
N=(635-35)/2=300
In general, the sum and difference problem can be solved (most of the time mentally) using
Larger number = (sum + difference)/2
Smaller number = (sum - difference)/2