Question 147102
(1 field)/(Sally's time) + (1 field)/(Brenda's time) = (1 field)/(time together)
{{{1/20 + 1/30 = 1/x}}}
Multiply both sides by {{{60x}}}
{{{3x + 2x = 60}}}
{{{5x = 60}}}
{{{x = 12}}}
Working together, they can plow the field in {{{12}}} hrs