8 b³ / 20 b³
May be easier to handle if you write it like this:
( 8 / 20 ) x ( b³ / b³ )
In the first fraction, divide top and bottom by 4, and it becomes ( 2 / 5 ).
In the second fraction, divide top and bottom by b³, and it becomes ( 1 / 1 ).
Now the problem is ( 2 / 5 ) x ( 1 / 1 ). This is 2/5 .
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Easier way:
8 b³ / 20 b³
Divide the top by 4b³ and it becomes 2 .
Divide the bottom by 4b³ and it becomes 5 .
Now the fraction is 2/5 .
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The important thing to understand:
(8 of anything) / (20 of the same thing) = 2/5
It doesn't matter if the things are cows, stones, leaves, or 'b³'s.