# math

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Can someone help me with this problem? multiply and simplify 4 1/5 x 10/21 x 9/20. I know after you simplify 4 1/5 it will be 21/5. What is the next step.

• math - ,

21/5 x 10/21 x 9/20. You can multiply all the numerators (top numbers) and then all the denominators (bottom numbers), then simplify. However,it is much easier to simplify before multiplying. To do this, find a number that will divide evenly into any numerator and any denominator. There is a 21 for a num. and a 21 for a den. so 21/21 = 1. Now you have 1/5 x 10/1 x 9/20. You have a 5 in a den and a 10 in a num so you could divide out the common 5 which is 1/1 x 2/1 x9/20.( Or you could look at the num 10 and the den 20 - dividing out the common 10. It does not matter which num and den you use as long as they have a common factor and are not both num or both den.)In 1/1 x 2/1 x 9/20, there is a common factor of 2 that can be divided into the num 2 and the den 20 leaving 1/1 x 1/1 x 9/10. Now multiplying all num then all den gives you 9/10. This is much easier to see when you write it on paper. If you don't understand let me know