20,4+5*(0,3а-4,2)=

To solve the expression \( 20.4 + 5 \cdot (0.3a - 4.2) \), follow the order of operations (parentheses, exponents, multiplication and division from left to right, and addition and subtraction from left to right).

1. Start with the multiplication: \[ 5 \cdot (0.3a - 4.2) = 5 \cdot 0.3a - 5 \cdot 4.2 = 1.5a - 21 \]

2. Now, substitute this back into the original expression: \[ 20.4 + (1.5a - 21) \]

3. Combine the constants: \[ 20.4 - 21 + 1.5a = -0.6 + 1.5a \]

Thus, the final expression is: \[ 1.5a - 0.6 \]