To find out how much water was lost from the pool, we need to subtract the amount of water left in the pool from the initial amount of water it had.
Initially, the pool was filled to \( \frac{3}{4} \) full. After playing all day, the pool was \( \frac{3}{10} \) full.
Let's perform the subtraction:
\[ \text{Water lost} = \text{Initial amount} - \text{Final amount} \]
\[ \text{Water lost} = \frac{3}{4} - \frac{3}{10} \]
To subtract these fractions, we need a common denominator. The least common multiple of 4 and 10 is 20.
Now, we convert each fraction:
\[ \frac{3}{4} = \frac{3 \times 5}{4 \times 5} = \frac{15}{20} \]
\[ \frac{3}{10} = \frac{3 \times 2}{10 \times 2} = \frac{6}{20} \]
Now we can subtract:
\[ \frac{15}{20} - \frac{6}{20} = \frac{15 - 6}{20} = \frac{9}{20} \]
So, the amount of water lost from the pool is \( \frac{9}{20} \).