Answers

2014-10-25T22:47:59+08:00
Let 'x' be the rate of Ann to finish the job
    'y' be the rate of Mary
    'z' be the rate of Naty
x(4) + y(4) = 1
 4x + 4y = 1    ----equation 1

x(6) + z(6) = 1
 6x + 6z = 1   ----equation 2

y(5) + z(5) = 1
 5y + 5z = 1   ----equation 3

multiply equation 1 with 6 and equation 2 with 4
4x + 4y = 1
24x + 24y = 6   -----equation 1'
6x + 6z = 1
24x + 24z = 4  -----equation 2'

subtract equation 2' from equation 1'
24y - 24z = 2
 dividing the whole equation by 2
 12y - 12z = 1  ------equation 4
multiply equation 3 with 12 and equation 4 with 5
5y + 5z = 1
60y + 60z = 12  ----equation 3'
12y - 12z = 1
60y - 60z = 5    -----equation 4'
add equation 4' and equation 3'
120y = 17
y = 17/120
substitute the value of y to equations 1 and 3
4x + 4y = 1
4x + 4(17/120) = 1
4x = 1 - 68/120
4x = 13/30
x = 13/120
5y + 5z = 1
5(17/120) + 5z = 1
5z = 1 - 85/120
5z = 7/24
z = 7/120
to get the time for each to finish the job alone, get the reciprocal of the rates.
y = 17/120
t(y) = 120/17 days
t(y) = 7.06 days
x = 13/120
t(x) = 120/13 days
t(x) = 9.23 days
z = 7/120
t(z) = 120/7 days
t(z) = 17.14 days
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2014-10-26T07:22:51+08:00

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A+M=4 \\ A+N=6 \\ M+N=5 \\ ======= \\\\ 2A+2M+2N=4+6+5 \\\\ 2(A+M+N)=15 \ |:2 \\\\ A+M+N=7.5\\ A+M=4 \\\\ =>N=7.5-4 \ \ \to\boxed{N=3.5 } \ (3 \ days \ and \ a \ half \ for \ Naty) \\\\\\ A+M+N=7.5 \\ A+N=6 \\ \\ => M=7.5-6 \ \ \to\boxed{M=1.5} \ (1 \ day \ and \ a \half \ for \ Mary) \\\\\\ A+M+N=7.5 \\ M+N=5 \\ \\ => A=7.5-5 \ \ \to\boxed{A=2.5} \ (2 \ days \ and \ a \ half \ for \ Ann)
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