20,4+5(0,3а-4,2)

To solve the expression \( 20.4 + 5(0.3a - 4.2) \), follow the order of operations:

1. Calculate the expression inside the parentheses: \[ 0.3a - 4.2 \]

2. Multiply the result by 5: \[ 5(0.3a - 4.2) = 5 \times 0.3a - 5 \times 4.2 = 1.5a - 21 \]

3. Add this result to 20.4: \[ 20.4 + (1.5a - 21) = 20.4 + 1.5a - 21 \]

4. Combine the constant terms: \[ 20.4 - 21 = -0.6 \]

Putting it all together, the final expression is: \[ 1.5a - 0.6 \]

So, \( 20.4 + 5(0.3a - 4.2) = 1.5a - 0.6 \).