21:(3x5)

To simplify the expression \( 21:(3 \times 5) \), first calculate the multiplication inside the parentheses:

\[ 3 \times 5 = 15 \]

Now replace the multiplication in the original expression:

\[ 21 : 15 \]

The colon (:) represents division, so you can write this as:

\[ \frac{21}{15} \]

To simplify \( \frac{21}{15} \), divide both the numerator and the denominator by their greatest common divisor, which is 3:

\[ \frac{21 \div 3}{15 \div 3} = \frac{7}{5} \]

Thus, the simplified expression is:

\[ \frac{7}{5} \text{ or } 1.4 \]