Grade 5 Mathematics
When multiplying decimals, multiply like whole numbers first, then count decimal places to place the decimal in your answer!
Unlike addition/subtraction, you do NOT line up the decimal points when multiplying.
Count decimal places in Factor 1 + Count decimal places in Factor 2
= Decimal places in the Product (Answer)
Find: 2.4 x 3
Step 1: Multiply like whole numbers
Step 2: Count & place decimal
Total: 1 + 0 = 1 decimal place
Answer: 7.2
Find: 1.5 x 2.3
Step 1: Multiply like whole numbers
Step 2: Count & place decimal
Total: 1 + 1 = 2 decimal places
Answer: 3.45
Check with estimation: 1.5 is about 2 and 2.3 is about 2. So 2 x 2 = 4. Our answer of 3.45 is close to 4. It makes sense!
Find: 0.6 x 0.4
6 x 4 = 24
Total: 1 + 1 = 2 decimal places
24 with 2 decimal places = 0.24
Answer: 0.24
Notice: When you multiply two numbers less than 1 (like 0.6 x 0.4), the answer is SMALLER than both numbers!
WRONG: Some students line up decimal points like they do for addition. This is only for adding/subtracting!
RIGHT: For multiplication, line up numbers on the RIGHT side. Ignore decimals until the end, then count decimal places to place it in your answer.
1. Find: 3.5 x 4
Decimal places: 3.5 has ____ + 4 has ____ = ____ total
Answer:
2. Find: 2.5 x 1.2
Decimal places: 2.5 has ____ + 1.2 has ____ = ____ total
Answer:
3. Find: 0.7 x 0.9
Decimal places: 0.7 has ____ + 0.9 has ____ = ____ total
Answer:
4. A bottle of juice costs $2.35. How much do 4 bottles cost?
Answer: $