You compress a folder containing 50 Word documents into a ZIP file. What savings can you realistically expect?
AAlmost none — ZIP cannot compress multiple files simultaneously
BSignificant savings — Word documents contain repetitive text patterns that ZIP can exploit to reduce file size dramatically
CExactly 50% reduction — ZIP always halves the size of any file
DNone — compression only works on image files
Text and document files have lots of redundant structure — repeated words, common phrases, predictable formatting patterns. ZIP's lossless compression exploits this redundancy through substitution (storing a short code for a repeated sequence instead of the sequence itself). A plain Word document often compresses to 20–50% of its original size. The key factor is unexploited redundancy: the more repetitive the data, the better compression works.
Question 2 Multiple Choice
A user zips a folder of JPEG vacation photos and is surprised to find the ZIP is nearly the same size as the original. What explains this?
AThe ZIP program malfunctioned and should be reinstalled
BJPEG files are already internally compressed by a specialized algorithm, leaving almost no redundancy for ZIP to exploit
CZIP cannot process image files — only text and document files
DThe folder contains too many files for ZIP to compress efficiently
This is the most important practical fact about compression: it only reduces size when unexploited redundancy remains. JPEG was specifically designed to compress photographic images by removing redundancy from pixel data — it does this far more efficiently than a general-purpose compressor like ZIP. When you add JPEGs to a ZIP file, ZIP finds almost no patterns to exploit and adds a small overhead of its own. The result is a ZIP that is roughly the same size as the original files, or even slightly larger.
Question 3 True / False
A TAR file is a compressed archive — it both bundles files together and compresses them.
TTrue
FFalse
Answer: False
TAR (Tape Archive) is a bundling format only — it packages multiple files and folders into a single file without any compression. Compression is applied separately, producing combined formats like .tar.gz (TAR bundled, then compressed with gzip) or .tar.bz2 (TAR bundled, then compressed with bzip2). ZIP, by contrast, does both operations simultaneously — it bundles and compresses in a single step. The TAR + gzip convention is common on Unix/Linux systems, which is why many downloads use the .tar.gz extension.
Question 4 True / False
Double-clicking a ZIP file on Windows or macOS fully extracts its contents so that the files inside behave exactly like normal files.
TTrue
FFalse
Answer: False
Most operating systems open ZIP files in a browsing mode that lets you see and even open individual files, but the contents remain inside the compressed archive rather than being fully extracted to a folder. Files opened directly from inside a ZIP may behave unexpectedly — edits may not be saved, programs may not find their associated files, and paths may not resolve correctly. To use ZIP contents reliably, you should right-click and choose 'Extract All' (Windows) or drag the contents to a folder (macOS), creating a proper copy on the filesystem.
Question 5 Short Answer
Why does compressing an already-compressed file (like a JPEG image or MP4 video) result in almost no size reduction?
Think about your answer, then reveal below.
Model answer: Lossless compression works by finding and encoding redundant patterns in data — repeated sequences that can be stored more compactly than by listing every byte. JPEG and MP4 were specifically designed to remove redundancy from image and video data using algorithms tuned to those media types. After JPEG or MP4 compression, almost no exploitable redundancy remains. When a general-purpose compressor like ZIP tries to compress the result, it finds no patterns to exploit and the compressed output is roughly the same size as the input — sometimes slightly larger due to compression overhead.
The key insight is that compression ratios multiply: if a format has already achieved a 10:1 compression ratio, a second pass at general compression gets you approximately 1:1 (no additional reduction). This is why ZIP a folder of mixed files produces dramatic savings on the Word documents and spreadsheets but almost none on the photos and videos. Understanding which file types already use internal compression prevents the common mistake of expecting ZIP to shrink already-compressed media.