Discover Mathematics P5: Term 1, Units 1–6, Review
GoalReview place value, rounding, adding and subtracting decimals, prime factors, the GCF and the LCM.
New words
prime number · a number with exactly two factors, 1 and itself, such as 7greatest common factor (GCF) · the biggest number that divides two numbers exactlyleast common multiple (LCM) · the smallest number that is a multiple of both numbers
Explanation
Decimals: each place is worth 10 times the place to its right. Compare from the left and line up the points. To round, look at the digit to the right of the place you keep: 5 or more rounds up. Add and subtract by lining up the points and filling gaps with zeros, and estimate to check. Factors: a prime number has exactly two factors. A factor tree splits a number into primes. The GCF is the biggest number that divides both numbers; the LCM is the smallest number that both numbers divide. Use the GCF to share into equal groups and the LCM to find when two things happen together.
Round 40.368 to the nearest tenth and to the nearest hundredth. The tenth digit is 3 and the next digit is 6, so round up: 40.4. The hundredth digit is 6 and the next digit is 8, so round up: 40.37.
Work out 12.75 + 8.6 − 4.325. Add: 12.75 + 8.6 = 21.35. Subtract: 21.35 − 4.325 = 17.025. Estimate: 13 + 9 − 4 = 18, which is close.
Find the GCF and LCM of 6 and 8. Factors of 6: 1, 2, 3, 6. Factors of 8: 1, 2, 4, 8. GCF = 2. Multiples of 6: 6, 12, 18, 24. Multiples of 8: 8, 16, 24. LCM = 24. The tree shows that 72 = 2 × 2 × 2 × 3 × 3.
Class activity
Number detective. Roll a die three times to build a number with a decimal point, for example 4.26. Write its expanded form, round it to the nearest tenth, and say which is greater: your number or another number made from the same digits (such as 4.62). Then pick two numbers under 20 and find their GCF and LCM.
Practice
1
Write 4,050.306 in words.
Four thousand fifty and three hundred six thousandths.
2
Round 7.485 to the nearest tenth and to the nearest hundredth.
