Question 315059
1.If dan makes 20 drinks in 5 minutes and Sue makes 20 drinks in 10 minutes and 
Jack makes 20 drinks in 15 minutes.
 How much time will it take all 3 of them working together to mix the 20 drinks?
:
let t = time required when they all worked together
:
Let the completed job = 1 (the mixing of 20 drinks)
:
{{{t/5}}} + {{{t/10}}} + {{{t/15}}} = 1 
Multiply by 30, results
6t + 3t + 2t = 30
:
11t = 30
t = {{{30/11}}}
t = 2.73 minutes, or 2 min 44 sec
:
:
Check solution using a calc
{{{2.73/5}}} + {{{2.73/10}}} + {{{2.73/15}}} =
.546 + .273 + .182 = 1.00