Answer: The required number of quarters in the collection is 11.
Step-by-step explanation: Given that a collection of 20 coins made up of only nickels, dimes and quarters has a total value of $3.35.
If the dimes were nickels, the nickels were quarters and the quarters were dimes, the collection of coins would have a total value of $2.75.
We are to find the number of quarters in the collection.
Let x, y and z represents the number of nickels, dimes and quarters respectively in the collection.
We will be using the following values of nickels, dimes and quarters in form of dollar :
1 nickel = $ 0.05, 1 dime = $ 0.10 and 1 quarter = $0.25.
Then, according to the given information, we have
![x+y+z=20\\\\\Rightarrow x=20-y-z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(i)\\\\0.05x+0.10y+0.25z=3.35\\\\\Rightarrow 5x+10y+25z=335\\\\\Rightarrow x+2y+5z=67~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(ii)\\\\0.05y+0.25x+0.10z=2.75\\\\\Rightarrow 5y+25x+10z=275\\\\\Rightarrow y+2z+5x=55~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(iii)](https://tex.z-dn.net/?f=x%2By%2Bz%3D20%5C%5C%5C%5C%5CRightarrow%20x%3D20-y-z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28i%29%5C%5C%5C%5C0.05x%2B0.10y%2B0.25z%3D3.35%5C%5C%5C%5C%5CRightarrow%205x%2B10y%2B25z%3D335%5C%5C%5C%5C%5CRightarrow%20x%2B2y%2B5z%3D67~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28ii%29%5C%5C%5C%5C0.05y%2B0.25x%2B0.10z%3D2.75%5C%5C%5C%5C%5CRightarrow%205y%2B25x%2B10z%3D275%5C%5C%5C%5C%5CRightarrow%20y%2B2z%2B5x%3D55~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28iii%29)
Substituting the value of x from equation (i) in equations (ii) and (iii), we have
![(20-y-z)+2y+5z=67\\\\\Rightarrow y+4z=47\\\\\Rightarrow y=47-4z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(iv)](https://tex.z-dn.net/?f=%2820-y-z%29%2B2y%2B5z%3D67%5C%5C%5C%5C%5CRightarrow%20y%2B4z%3D47%5C%5C%5C%5C%5CRightarrow%20y%3D47-4z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28iv%29)
and
![y+2z+5(20-y-z)=55\\\\\Rightarrow -4y-3z=-45\\\\\Rightarrow y=\dfrac{45-3z}{4}~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(v)](https://tex.z-dn.net/?f=y%2B2z%2B5%2820-y-z%29%3D55%5C%5C%5C%5C%5CRightarrow%20-4y-3z%3D-45%5C%5C%5C%5C%5CRightarrow%20y%3D%5Cdfrac%7B45-3z%7D%7B4%7D~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~%28v%29)
Comparing the values of y from equations (iv) and (v), we get
![47-4z=\dfrac{45-3z}{4}\\\\\Rightarrow 188-16z=45-3z\\\\\Rightarrow 16z-3z=188-45\\\\\Rightarrow 13z=143\\\\\Rightarrow z=\dfrac{143}{13}\\\\\Rightarrow z=11.](https://tex.z-dn.net/?f=47-4z%3D%5Cdfrac%7B45-3z%7D%7B4%7D%5C%5C%5C%5C%5CRightarrow%20188-16z%3D45-3z%5C%5C%5C%5C%5CRightarrow%2016z-3z%3D188-45%5C%5C%5C%5C%5CRightarrow%2013z%3D143%5C%5C%5C%5C%5CRightarrow%20z%3D%5Cdfrac%7B143%7D%7B13%7D%5C%5C%5C%5C%5CRightarrow%20z%3D11.)
Thus, the required number of quarters in the collection is 11.