Decimal and binary
Decimal units step by 1,000: a kilobyte is 1,000 bytes, a megabyte 1,000,000. Binary units step by 1,024 and have an i in the name: a kibibyte (KiB) is 1,024 bytes, a mebibyte (MiB) 1,048,576. Drive makers use decimal units; Windows counts in binary but labels the result KB, MB and GB, which is why a 1 TB drive shows as 931 GB. That 931 is GiB: 1 TB is 931.32 GiB.
Bits and bytes
A byte is 8 bits. Internet speeds are sold in megabits (Mb, small b) per second and file sizes are given in megabytes (MB, big B), so a 100 Mb/s line moves at most 12.5 MB each second. The units here are amounts, not rates: 100 megabits is 12.5 megabytes.
Exact to the last digit
Every factor here is a power of 2 or 10, so whole-number answers are worked out exactly with no rounding: 1 EiB is 1,152,921,504,606,846,976 bytes, every digit right. Fractions are rounded to twelve significant figures.
Questions
How many MB are in a GB?
1,000 in decimal units, and 1,024 MiB in a GiB in binary ones. Which one a phone plan or cloud service means is usually in its small print.
Is a kilobit 1,000 or 1,024 bits?
1,000 here. Kilobits, megabits and gigabits are decimal, as network speeds are.