Question 1155786
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M + A = 3900

M - A = 1100

--------------------Add the equations


2M = 3900 + 1100 = 5000  ===>  M = 2500  ===>  Manu worked 2500/250 =  10 days.


Subtract the equation

    2A = 3900-1100 = 2800  ====> A = 1400  ====>  Ahmed worked  1400/200 = 7 days.
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Solved (in a simplest way).