Solve innermost parenthesis

Evaluate the innermost parenthesis by first performing the division inside it: $12 \div 4 = 3$, then subtract: $10^{-3} = 7$.

$10^{-12} \div 4 = 10^{-3} = 7$

Simplify the next level of brackets

Substitute the result back into the middle bracket: $(80 \div 4 - 7 \div 7)$. First, $80 \div 4 = 20$ and $7 \div 7 = 1$. Then, subtract: $20 - 1 = 19$.

$80 \div 4 - 7 \div 7 = 20 - 1 = 19$

Calculate the outer square bracket

Substitute 19 into the outer bracket: $30 - 19 = 11$.

$30 - 19 = 11$

Final subtraction

Subtract the result of the brackets from 50: $50 - 11 = 39$.

$50 - 11 = 39$