We have two children of Grade 3 and 4 and we have to find the probability of these two.

$$Probabilityofgrade3child=\frac{1}{20}$$

and

$$Probabilityofgrade4child=\frac{1}{15}$$

$$\therefore Probabilityofbothchildren=\frac{1}{15}\times \frac{1}{20}=\frac{1}{300}$$