Mental Maths (Additions / Subtractions of Fractions)

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In this post we shall discuss How to Add / Subtract Fractions... Have a look at the following Problems.

Before reading this post, Have a look at How to Improve your Calculation Speed ?

• (4/9) + (13/18) + (7/54) = ?

• The LCM of the denominators 9, 18 and 54 is 54. (You should calculate LCM mentally. As 54 is divisible by 9, 18, and 54 the LCM is 54]. After finding the LCM, you should concentrate on Calculating the Numerators.
• As 9 has to be multiplied by 6 to get 54, the numerator 4 is multiplied by 6 i.e. 4 X 6 = 24.
• Similarly 13 X 3 = 39 and 7 X 1 = 7.
• So, (4/9) + (13/18) + (7/54) = (24/54) + (39/54) + (7/54)

                                        = (24+39+7) / 54  =  70/54

                                              

                                        = 35/27

• (7/18) - (11/24) + (13/36) = ?

• The LCM of the denominators 18, 24 and 36 is 72
• This 36 is divisible by 18, so the LCM of 18 and 36 is 36. 
• To find the LCM of 24 and 36, take the larger number i.e. 36 and its multiples 72, 108, etc. 36 is not divisible by 24. So, LCM is not 36. 72 is divisible by 24. So the LCM is 72. The denominator of the resultant factor is 72.

                              =  (7/18) - (11/24) + (13/36)

                              =  (28/72) - (33/72) + (26/72) = 21/72

                              =  7/24


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