Calculate Total Distance: Adding 1.2, 2.1, and 1.7 Kilometers in a Three-Day Journey

Decimal Addition with Column Alignment

Andy goes on a three-day trip.

On day one, he travels 1.2 1.2 km.

On day two, he travels 2.1 2.1 km.

On day three, he travels 1.7 1.7 km.


How many kilometres does Andy travel in total over the three days?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Calculate the total km walked
00:12 We will use long addition for calculation
00:20 Each time we'll add and substitute accordingly
00:26 We'll place the units and remember 1 above
00:31 We'll add 1 to this calculation
00:39 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Andy goes on a three-day trip.

On day one, he travels 1.2 1.2 km.

On day two, he travels 2.1 2.1 km.

On day three, he travels 1.7 1.7 km.


How many kilometres does Andy travel in total over the three days?

2

Step-by-step solution

To work out how many kilometres Andy walks in total, we need to add up the distances he walks each day in the following manner:

1.2+2.1+1.7 1.2\\+2.1\\+1.7

First we will add the digits after the decimal point accordingly:

2+1+7=10 2+1+7=10

Since these add up to 10, we can replace the tenths digit with a 0 and add a units digit to the number.

Now, simply add up the units digits of the number (the original digits before the decimal place plus our new one):

1+1+2+1=5 1+1+2+1=5

In total, Andy walks 5 km (5.0 km).

3

Final Answer

5 5

Key Points to Remember

Essential concepts to master this topic
  • Rule: Line up decimal points vertically when adding decimal numbers
  • Technique: Add tenths first: 0.2 + 0.1 + 0.7 = 1.0
  • Check: Verify by estimation: 1 + 2 + 2 ≈ 5 kilometers ✓

Common Mistakes

Avoid these frequent errors
  • Adding decimal digits without proper alignment
    Don't add 12 + 21 + 17 = 50 by ignoring decimal places! This gives 50 instead of 5, ten times too large. Always line up decimal points vertically and add each place value column separately.

Practice Quiz

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Which decimal number is greater?

FAQ

Everything you need to know about this question

Why do I need to line up the decimal points?

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Lining up decimal points ensures you're adding like place values: tenths to tenths, units to units. Without alignment, you might add a tenth to a unit, giving the wrong answer!

What do I do when the tenths add up to more than 9?

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When tenths exceed 9 (like 2+1+7=10), write down 0 in the tenths place and carry 1 to the units column. This is called regrouping or carrying over.

Should my final answer have a decimal point?

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Yes, but if your answer is a whole number like 5.0, you can write it as either 5 or 5.0 - both are correct!

How can I check if my decimal addition is right?

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Try rounding each number to the nearest whole number first: 1+2+2=5. Your exact answer should be close to this estimate!

What if the decimals have different numbers of digits?

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Add zeros as placeholders! For example, treat 1.2 as 1.20 when working with 1.75. This makes alignment easier and prevents mistakes.

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