Answer: [tex]12\ \text{min}[/tex]
Step-by-step explanation:
Given
It takes 20 min and 30 min for a person and his friend to paint a wall individually i.e.
In 1 min they cover [tex]\frac{1}{20}[/tex] part and [tex]\frac{1}{30}[/tex] part of wall respectively.
If the work together, 1 minute work is
[tex]\Rightarrow \dfrac{1}{20}+\dfrac{1}{30}\\\\\Rightarrow \dfrac{3+2}{60}\\\\\Rightarrow \dfrac{5}{60}[/tex]
Time required will be reciprocal of it
[tex]\Rightarrow \dfrac{60}{5}=12\ \text{min}[/tex]
