Question 760048
Let A's rate of working = {{{ R[a] }}}
B works 3 times faster, so
B's rate of working = {{{ 3R[a] }}}
Their rate working together is ( 1 job ) / ( 9 days )
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If you add their rates of working, you get
their rate working together
{{{ R[a] + 3R[a] = 1/9 }}} 
{{{ 4R[a] = 1/9 }}}
{{{ R[a] = 1/36 }}}
{{{ 3R[a] = 3/36 }}}
{{{ 3R[a] = 1/6 }}}
Working alone, A will finish the job in 36 days
check:
{{{ R[a] + 3R[a] = 1/9 }}} 
{{{ 1/36 + 3/36 = 1/9 }}} 
{{{ 4/36 = 1/9 }}}
{{{ 1/9 = 1/9 }}}
OK