Running at the same constant rate, 6 identical machines can produce a total of 180 bottles per hour. How many bottles could 15 such machines produce on 30 minutes ?

Answer: Option A

Let the required number of bottles be x
More machines, More bottles produced (Direct proportion)
Less time, Less bottles produce (Direct proportion)
$$\begin{array}{c}\text{Machines 6}:15\text{hfill}\\ \text{Times 60}:30\text{hfill}\end{array}\}::180:x$$
$$\begin{array}{rl}& \therefore 6\times 60\times x=15\times 30\times 180\\ & \iff x=\frac{\left(15\times 30\times 180\right)}{\left(6\times 60\right)}\\ & \iff x=225\end{array}$$