Question 1155509
A and B  can paint a wall in 2 hours
A and C  can paint the wall in 3 hours
B and C can paint the wall in 4 hours
How long will it take to paint the wall if all 3 work together 
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{{{1/A + 1/B  = 1/2}}}
{{{1/A + 1/C  = 1/3}}}
{{{1/B + 1/C  = 1/4}}}
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Sub r, s & t for 1/A, 1/B & 1/C respectively.
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{{{r + s  = 1/2}}}
{{{r + t  = 1/3}}}
{{{s + t  = 1/4}}}
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s = 1/2 - r
{{{s + t  = 1/4}}}
{{{1/2 - r + t  = 1/4}}}
{{{-r + t  = -1/4}}}
{{{r - t  = 1/4}}}
{{{r + t  = 1/3}}}
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Add
2r = 7/12
r = 7/24
A = 24/7
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Solve for s & t, then B & C.
Can you do the rest?