Three and a half pounds of peaches cost $1.68 
How much will two and a half pounds cost? 
please put explanation thank you

3 1/2 is to $1.68 as 2 1/2 is to $_____?

Write 3 1/2 as 3.5 and 2 1/2 as 2.5

3.5 is to $1.68 as 2.5 is to $_____?

Replace the words "is to" by " / "
Replace the word "as" by " = "
Replace the $____ by x

3.5/1.68 = 2.5/x

Cross multiply. 
That is, multiply the 3.5 times the x,
getting 3x, and put that on the left 
side of an equal sign  

    3.5x =   

Now multiply the 1.68 by the 2.5, 
getting 4.2 and put that on the right
side of the equal sign

    3.5x = 4.2

Now divide both sides by 3.5

       x = 4.2/3.5

       x = 1.2 or $1.20

Edwin