12 monkeys can eat 12 bananas in 12 minutes. In how many minutes can 4 monkeys eat 4 bananas ?

$$\eqalign{ & {\text{Let the required time}} = {\text{T}} \cr & \Rightarrow \frac{{{{\text{m}}_{\text{1}}} \times {{\text{d}}_{\text{1}}} \times {{\text{t}}_{\text{1}}}}}{{{{\text{w}}_{\text{1}}}}}{\text{ = }}\frac{{{{\text{m}}_{\text{2}}} \times {{\text{d}}_{\text{2}}} \times {{\text{t}}_{\text{2}}}}}{{{{\text{w}}_{\text{2}}}}} \cr & \Rightarrow \frac{{12 \times 12}}{{12}} = \frac{{4 \times {\text{time}}}}{4} \cr & \Rightarrow {\text{Time}} = 12\operatorname{minutes} \cr} $$