15.6.3 Definition and properties of probability

Problem: The central accounting department of the corporation received packs of receipts for verification and processing. \( 90 \% \) of the packs were found to be satisfactory: they contained \( 1 \% \) of incorrectly formalized receipts. The rest \( 10 \% \) of the receipts were found to be dissatisfying, i.e., they contained \( 5 \% \) incorrectly formalized receipts. What is the probability that a randomly chosen receipt will be incorrectly formalized?