Question 132051
Let {{{x}}} = Angela's time working alone
Add the rates at which each works to 
get the rate of working together:
Note that if Brett can groom the horses in {{{50}}} min, and he works
alone for {{{10}}} min, he does 1/5 of the job, leaving 4/5 to be done
by both working together
(1 horse-grooming job)/(Angela's tme) + (1 horse-grooming job)/(Brett's time) =
(4/5 horse-grooming job done)/(time working together)
{{{1/50 + 1/x = (1 - (1/5))/ 15}}}
{{{1/50 + 1/x = (4/5)/15}}}
multiply both sides by {{{15*50*x}}}
{{{15x + 15*50 = (4/5)*50*x}}}
{{{15x + 750 = 40x}}}
{{{25x = 750}}}
{{{x = 30}}}
It would take Angela 30 min working alone
check:
{{{1/50 + 1/x = (4/5)/15}}}
{{{1/50 + 1/30 = (4/5)/15}}}
multiply both sides by {{{30*50}}}
{{{30 + 50 = (4/5)(30*50)/15}}}
{{{80 = (4/5)(100)}}}
{{{80 = 80}}}
OK