Question 1158785
The rate of each in number of cycles per seconds
Elroy  {{{1/20}}}
Herbert {{{1/30}}}


Time for 15 cycles Elroy and Herbert together:
{{{(1/20+1/30)x=15}}},  {{{LCD}}}{{{is}}}{{{60}}}.

{{{60(1/20+1/30)x=60*15}}}

{{{(3+2)x=60*15}}}

{{{5x=60*15}}}

{{{x=highlight(180)}}}{{{seconds}}}