math
posted by Anonymous .
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.

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