Question 878209
Add their rates of working
Together:
( 1 program ) / ( 15 hrs )
John alone:
( 1 program ) / ( t hrs )
Mary alone:
( 1 program ) / ( t - 3 )
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{{{ 1/t + 1/( t-3 ) = 1/15 }}}
Multiply both sides by {{{ 15*t*( t-3 ) }}}
{{{ 15*( t-3 ) + 15t = t*( t-3 ) }}}
{{{ 15t - 45 + 15t = t^2 - 3t }}}
{{{ t^2 - 33t + 45 = 0 }}}
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Solve for {{{ t }}} with quadratic formula
then find {{{ t }}} and {{{ t - 3 }}}