Adding & Subtracting Decimals — Worksheet & Memo

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Grade 8 · Decimal Fractions

Adding & Subtracting Decimals

MARKING GUIDELINE

Marks

Duration

50 minutes

Questions

Name:

Class:

Date:

Mark

/ 33

Instructions and Information

Answer ALL the questions in this question paper.
Answer QUESTION 1 by circling the letter (A–D) in the answer grid at the end of that section.
Show ALL calculations clearly.
Show all units where applicable.
Number the answers correctly according to the numbering system used in this question paper.
A non-programmable calculator may be used, unless stated otherwise.
Write neatly and legibly.

Question 1

[10 MARKS]

Four options are given as possible answers to the following questions. Choose the correct answer and circle the letter (A–D) in the grid at the end of this section. If you want to change your choice, put a cross through the wrong letter and circle your new choice.

1.1
3,4 + 12,75 = …
(1)
A)16,15✓
B)4,675
C)15,15
D)16,79
Answer: A — Line up the COMMAS: 3,40 + 12,75.
- B — lined up the last digits instead of the commas
- C — lost a unit in the carrying
- D — added the digits without aligning place value
1.2
20 − 6,45 = …
(1)
A)14,55
B)13,55✓
C)14,45
D)26,45
Answer: B — 20,00 − 6,45 = 13,55.
- A — did not borrow from the units
- C — subtracted the hundredths the wrong way
- D — ADDED instead of subtracting
1.3
When adding decimals you must line up the …
(1)
A)last digits
B)commas✓
C)first digits
D)longest numbers
Answer: B — Lining up the commas lines up the place values.
- A — that is what causes 3,4 + 12,75 to come out as 4,675
- C — the whole-number parts can be different lengths
- D — length has nothing to do with it
1.4
A pipe 2,5 m long has 0,85 m cut off. The piece left is …
(1)
A)1,65 m✓
B)2,35 m
C)1,75 m
D)3,35 m
Answer: A — 2,50 − 0,85 = 1,65 m.
- B — subtracted only the tenths
- C — borrowed incorrectly
- D — ADDED the two lengths
1.5
0,7 + 0,3 = …
(1)
A)0,10
B)1✓
C)0,1
D)0,21
Answer: B — 7 tenths plus 3 tenths is 10 tenths, which is one whole.
- A — wrote ten tenths as 0,10 instead of carrying
- C — same slip, written as 0,1
- D — multiplied instead of adding
1.6
Thandi buys items for R12,60, R8,45 and R23,95. Her total is …
(1)
A)R44,00
B)R45,00✓
C)R43,90
D)R45,10
Answer: B — 12,60 + 8,45 + 23,95 = 45,00.
- A — lost a rand in the carrying
- C — did not carry from the cents
- D — carried twice
1.7
Which one of the following is CLOSEST to 1?
(1)
A)0,89
B)0,98✓
C)1,2
D)0,9
Answer: B — The differences are 0,11 ; 0,02 ; 0,2 and 0,1.
- A — 0,11 away
- C — 0,2 away
- D — 0,1 away
1.8
She pays with R50 for goods costing R38,75. Her change is …
(1)
A)R12,25
B)R11,25✓
C)R12,75
D)R11,35
Answer: B — 50,00 − 38,75 = 11,25.
- A — did not borrow from the tens
- C — subtracted the cents the wrong way round
- D — borrowed incorrectly in the cents
1.9
6,08 + 0,9 = …
(1)
A)6,17
B)6,98✓
C)7,08
D)6,89
Answer: B — 6,08 + 0,90 = 6,98 — the 9 is TENTHS, not hundredths.
- A — added the 9 as hundredths
- C — added the 9 as units
- D — reversed the digits of the answer
1.10
Which calculation gives an answer of exactly 5?
(1)
A)4,5 + 0,4
B)3,75 + 1,25✓
C)2,8 + 2,3
D)1,95 + 3,15
Answer: B — 3,75 + 1,25 = 5,00.
- A — gives 4,9
- C — gives 5,1
- D — gives 5,1

Answer grid — marking guideline

1.1	A	B	C	D
1.2	A	B	C	D
1.3	A	B	C	D
1.4	A	B	C	D
1.5	A	B	C	D
1.6	A	B	C	D
1.7	A	B	C	D
1.8	A	B	C	D
1.9	A	B	C	D
1.10	A	B	C	D

Question 2

[11 MARKS]

Calculate the following WITHOUT using a calculator. Show ALL your working.

2.1
20 − 6,45 − 2,8
(4)
Write 20 as 20,00 and work from left to right (1)
20,00 − 6,45 = 13,55 (2)
13,55 − 2,80 = 10,75 (1)
2.2
(4,6 + 2,35) × 0,2
(4)
Brackets FIRST: 4,60 + 2,35 = 6,95 (2)
6,95 × 0,2 (1) = 1,39 (1)
2.3
Estimate 19,7 + 4,2 by rounding each number to the nearest whole number, then give the exact answer.
(3)
20 + 4 = 24 (1)
Exactly: 23,9 (1)
The estimate is close, so the answer is reasonable (1)

Question 3

[12 MARKS]

Ann buys three items at a shop: R32,40, R15,85 and R9,99. She pays with a R100 note.

3.1
Calculate the total cost of the three items.
(3)
32,40 + 15,85 + 9,99 (2) = R58,24 (1)
3.2
Calculate the change Ann receives.
(3)
100,00 − 58,24 (2) = R41,76 (1)
3.3
A plank is 2,4 m long. A piece of 0,85 m and a piece of 0,6 m are cut off. Calculate the length of the plank that is left.
(4)
2,40 − 0,85 = 1,55 m (2)
1,55 − 0,60 (1) = 0,95 m (1)
3.4
Explain why the decimal commas must be lined up before adding.
(2)
Lining up the commas lines up the place values (1); tenths must be added to tenths and hundredths to hundredths (1)

TOTAL: 33 marks

This question paper consists of 3 questions.