A ribbon is $3m$ long. I need $8$ pieces of ribbon each $40cm$ long. How many centimetres shorter is the ribbon than the length I need?

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Hint:In this question we are provided with two measurement values but look clearly that their units are different. Multiply the length of the small piece needed with the number of such pieces needed. Then take the difference between the above calculated value and the total length of the ribbon available.

Complete step-by-step answer:
Let us note down the given values initially,
Total length of the ribbon is $3m$
Number of pieces required are $8$
Length of each piece is $40cm$
From the above data, if you look carefully
The total length of the ribbon is given in meters and the length of each piece is given in centimetres.
So, let us change the value of the total length of the ribbon from meters to the centimetres.
As we know that $1m = 100cm$,
Total length of the ribbon $ = 3 \times 100$
    $ = 300cm$
So, we got the total length of the ribbon also in centimetres.
Now let us check how many ribbons can we make using the available ribbon.
For that we need to divide the total length of the ribbon with the length of each piece.
So, the number of ribbons that can be made $ = \dfrac{{300}}{{40}}$
         $ = 7.5$
So, we can only make $7$ ribbon pieces and left with some amount of ribbon that is not sufficient to make a new ribbon piece.
Now, calculate the length of the ribbon required to make $8$ ribbon pieces $ = 8 \times 40$
                                                     $ = 320cm$
But we have only $300cm$ of the ribbon available.
The extra ribbon required $ = 320 - 300$
                   $ = 20cm$
Hence, we need more $20cm$ of ribbon to make $8$ ribbon pieces.

Note:Units play a very important role in this kind of problem. If we provided all the measurements with the same units, then there would be no problem at all.Students should remember standard measurements and unit conversions in length,mass and time for solving these types of problems.