In a factory, machine A produces $30\%$ of total output, machine B produces $25\%$ and the machine $C$ produces the remaining output. The defective items produced by machines $A, B$ and $C$ are $1\%, 1.2\%, 2\%$ respectively. An item is picked at random from a day's output and found to be defective. Find the probability that it was produced by machine $B$ ?