math
posted by allen on .
At the school store, 5 pens sell for $1.25. Which proportion will help find the cost of 12 pens?

What are your choices?
What do you think the answer is? 
a. 5/12=1.25/x
b. 5/12=x/1.25
c. 5/x=12/1.25
d. x/5=12/1.25 
I say its c

5 pens / 12 pens = 1.25 dollars/x dollars
It helps to write the units, pens and dollars.
so
I claim a is correct
although I myself would have written the same thing as
5 pens/1.25 dollars = 12 pens/x dollars
which comes out the same 
5 pens cost 1.25$
1.25/5=0.25$ Price of one pen
12*0.25=3$ Price of 12 pens