Set up an equation where you have the total oranges (o) on top and the cost (c) on the bottom. So you will have two fractions equaling each other. So you will have o/c=o/c. Now you just have to replace the variables with what you know. 9 oranges= $3.60, and you want to know how much 4 oranges cost. Since the cost per orange is the same, no matter how many oranges you buy if you divide the amount of oranges by the price you will always get the same number. So replace the variables with the numbers you know and you get,
9/$3.60=4/c
Now to solve an equal fraction problem, simply cross multiply (multiply the two numbers diagonal so 4 and 3.6), and divide (take the multiplied number and divide by the number remaining which will be the number diagonal from the variable) which gives you $1.60 for four oranges.
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