Question 1137526
Phong,  {{{1/x}}}

Burt,  {{{1/(3x)}}}

{{{1/x+1/3x=1/90}}}

-

{{{3/3x+1/3x=1/90}}}

{{{4/(3x)=1/90}}}

{{{3x/4=90}}}

{{{x=90(4/3)}}}

{{{highlight(x=120)}}}{{{minutes}}}