You can set up a proportion to solve for the percentage of the coins that are pennies. Of course, there are alternate methods as well, but this is one method. First, you define the percentage of the coins that are pennies to be equal to a variable, such as x. Next, you write 240/600 = x/100, due to how "x" is the amount out of 100 (since per cent is for every cent (out of 100)), and 240 would correspond to x while 600 would correspond to 100. This proportion may also be written as 100/x = 600/240, or 240/x = 600/100. In order to solve for x, you use cross-products, or you multiply each denominator by the numerator of the other fraction. You will be left with a numerical value that's equal to a number times x, and then you divide both sides of the equation by the coefficient of x in order to isolate x. As a result, you will have the percentage of the coins that are pennies to be your answer. Remember to write the units for every numerator and denominator in your proportion.
It works because 7 x 9 x 10 is the same thing as 7 x 90, since 9 x 10 is just 90. So basically he is just writing more but the answer will still be the same.
