If in a pond there were 50 water lily plants last year and through...

Initial population of water lily plants, $N = 50$

Number of new plants added through reproduction, $\Delta N = 15$

Time period, $\Delta t = 1$ year

Birth rate is calculated as the number of new individuals produced per unit of the initial population per unit time.

Birth rate $= \dfrac{\Delta N}{N \times \Delta t}$

Birth rate $= \dfrac{15}{50 \times 1} = 0.3$ offspring per water lily per year.

Answer: $0.3$