Friendly Products
Part of the Numbers & Operations section of Coddy's Pre-Algebra journey. Lesson 5 of 60.
Products can be regrouped just like sums: multiplying numbers in a different order never changes the result. That lets you hunt for friendly pairs. 2 × 13 × 5: the 2 and the 5 make 10, so the first line is 10 × 13.
Multiplying by 10 is easy, because it just puts a zero on the end: 10 × 13 is 130. Pairs that make 10, 100 or 1000 turn a hard-looking product into a quick one.
The pairs worth remembering are 2 with 5, which make 10, and 4 with 25, which make 100. Spotting them is not a trick for tests only; it is how people who are fast at arithmetic actually think.
Challenge
Work out 4 × 9 × 5. Pair the friendly numbers first, then finish.
Try it yourself
Simplify the expression
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Numbers & Operations
1Working on the Board
Your First AnswerType the Next LineOne Line at a TimeDots and BarsFriendly ProductsRecap: Board Work4Adding Integers
Adding Two NegativesDifferent SignsZero PairsAdding Several IntegersRecap: Adding Integers2Order of Operations
Multiply Before AddingLeft to RightParentheses FirstBrackets Inside BracketsThe Fraction Bar GroupsRecap: Order of Operations5Subtracting Integers
Add the OppositeSubtracting a NegativePlus and Minus in a RowDistance Between NumbersRecap: Subtracting Integers