Work and Algebra

In one hour, sandhya and savitri can do 3 times the work.

Similarly, sandhya and Maya can do 4times the work.

And savitri and Maya can do 5 times the work.

Hence a team of 2 people each can do 12 times the work. I.e, 3 members together can do 6 times the work in an hour.

Therefore sandhya can do the work in 6-5=1 hour... Or 60 minutes.

Let Sandhya be $x$ , Savitri be $y$ and Maya be $x$ .

One hour's work =

$\frac { 1 }{ x } +\frac { 1 }{ y } =\frac { 1 }{ 20 } \\ \frac { 1 }{ x } +\frac { 1 }{ z } =\frac { 1 }{ 15 } \\ \frac { 1 }{ y } +\frac { 1 }{ z } =\frac { 1 }{ 12 }$

On subtracting fraction 3 from 1 and then adding fraction 2 to the answer, we get

$\frac { 2 }{ x } =\frac { 1 }{ 30 } \\ x=60$