Number of minutes in 3/4 of an hour is :
n = 3/4 × 60 = 45 minutes.
Now, If it continues to rain at that rate for 15 more minutes :
So, time taken = 45 + 15 min = 1 hour.
Let, rain gauge is filled x fraction in 1 hour.
So,
[tex]\dfrac{3}{4}x = \dfrac{2}{5}\times 1\\\\x = \dfrac{8}{15}[/tex]
Hence, this is the required solution.