Real-world decimal problems

Decimals show up everywhere in real life: prices at the store, distances on a map, weights on a scale, and times on a stopwatch. In this project lesson you will use the four operations on decimals to solve problems people meet every day.

Step 1 — Read and Plan

1. Read the problem twice — slowly. Picture what is happening.
2. Underline the numbers and the question being asked.
3. Decide on the operation: total or sum means add; left over or change means subtract; groups of or per item means multiply; share equally or how many fit means divide.
4. Estimate first by rounding. Your final answer should be close to your estimate.

Adding and Subtracting Decimals

• Line up the decimal points, then add or subtract column by column from right to left.
• Keep the decimal point in the same column in your answer.
• Add zeros at the end if you need to make the place values match (3.5 becomes 3.50 to subtract 1.27).
• Money example: $4.99 + $3.20 = $8.19 — the total of two items at the till.

Multiplying Decimals

• Multiply the numbers as if there were no decimal points.
• Count the total decimal places in BOTH factors.
• Place the decimal in the answer so it has that same number of decimal places.
• Shopping example: 4 kg of apples at $1.25 per kg = 4 x 1.25 = $5.00.

Dividing Decimals

• If the divisor has a decimal, multiply both numbers by 10, 100, or 1,000 to make the divisor a whole number.
• Then divide as usual and bring the decimal straight up into the answer.
• Measurement example: 1.5 m of rope cut into 0.3 m pieces = 1.5 / 0.3 = 15 / 3 = 5 pieces.
• Sharing example: $12.60 split between 3 friends = 12.60 / 3 = $4.20 each.

Estimate to Check

• Round each decimal to the nearest whole number, then redo the operation in your head.
• If the estimate is far from the answer, recheck your decimal point.
• Example: 2.49 + 1.75 + 3.20 + 4.99 rounds to 2 + 2 + 3 + 5 = 12, so 12.43 looks right.