The partial products algorithm allows anyone (even students who are "bad" at math) to learn how to multiply 2- and 3-digit numbers! It is also easy to see why it works. You don't have to be a math whiz!
Partial products means that we multiply in part-by-part. For example, 7 × 534 is done as 7 × 500, 7 × 30, and 7 × 4, and lastly those parts are added.
This topic is also explained (along with practice problems) in my book Math Mammoth Multiplication 2 – available both as an inexpensive download ($5.10) and as a printed copy.
Remainder in division-related word problems — video lesson
Math Mammoth Multiplication 2 — a workbook with worksheets and lessons for long division, remainder, factors, and primes
Math Mammoth Grade 4 curriculum