Question 1153426
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Let x be the number of computers under the question (a).


Then your equation is


    {{{1800000/(x-5)}}} - {{{1800000/x}}} = 4000.


having in EACH side the price difference for one single computer.


For simplicity, take off 3 zeroes in each term of the equation, measuring the money in thousands of " sh ".


    {{{1800/(x-5)}}} - {{{1800/x}}} = 4.


You can reduce this equation to quadratic equation and to complete the solution by the Algebra way.


But you can solve it MENTALLY, by noticing that 45 and 50 are two factors of 1800, that differ by 5 units and give the quotients 

{{{1800/45}}} = 40 and {{{1800/50}}} = 36 that differ by 4.


So, your answer is 50:  he bought 50 computers, finally.


<U>CHECK</U>.  {{{1800/45}}} - {{{1800/50}}} = 40 - 36 = 4.
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Solved.


Do the rest of the assignment on your own.