Introduction
The expression 779 X 0.21 is a practical starting point for learning decimal multiplication. This beginner-friendly guide walks you through clear, step-by-step ideas to multiply a large whole number by a decimal, helping you build confidence with each simple concept you apply.
Key Points
- Think of 0.21 as the fraction 21/100 to understand why the final result includes a decimal shift.
- Split 0.21 into 0.2 and 0.01 to form easy partial products: 779 × 0.2 and 779 × 0.01.
- The number of decimal places in the product equals the total decimal places in the factors (0.21 has two decimals, 779 has none).
- Use estimation first to sanity-check your answer: 779 × 0.21 is near 800 × 0.21 = 168.
- Practice with similar pairs to increase speed and accuracy over time.
How to compute 779 X 0.21
One method uses fractions: 0.21 = 21⁄100, so 779 × 0.21 = (779 × 21) / 100. Multiplying 779 by 21 gives 16,359, and dividing by 100 yields 163.59.
Another practical approach is to split the decimal: 779 × 0.21 = 779 × (0.2 + 0.01) = 779 × 0.2 + 779 × 0.01 = 155.80 + 7.79 = 163.59. Both paths lead to the same result, reinforcing the concept.
Practice and quick checks
To get comfortable with decimals, practice by breaking other decimals into parts, estimate before calculating, and double-check decimal placement after you finish.
How do you multiply 779 by 0.21 step by step?
+Convert 0.21 to a fraction and multiply, then convert back to decimal: (779 × 21) / 100 = 16,359 / 100 = 163.59. Alternatively, split 0.21 into 0.2 + 0.01 and sum the partial products: 779 × 0.2 = 155.80 and 779 × 0.01 = 7.79, total 163.59.
Why does the decimal place in the result matter here?
+Because 0.21 has two decimal places, the product must reflect those two decimals. When you multiply by a whole number like 779, the decimal location is determined by the total decimal places in the factors. In this case, the result will have two decimal places: 163.59.
Can I apply the same method to any decimal?
+Yes. You can either convert the decimal to a fraction and multiply, or break the decimal into easier parts and add the partial products. The key ideas are keeping track of decimal places and ensuring the final sum reflects those places accurately.
