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1.Write down two prime numbers much that their sum is divisible by both 4 and 10.

The prime numbers are 3 and 17

Sum of 3 and 17 = 3+17 = 20
20/4 = 5, 20/10=2


2. Use all the numbers 10,20,30,40 and 50with the arithmetic operations(+,-,X,/)and brackets ,write down an expression with a result of 0.

The expression is: (40/20)X30 – (50 + 10)

(40/20)X30 – (50 + 10)
2 X 30 - 60
60 – 60 = 0

Use up +, -, X, / and two brackets as required.


3. Give three numbers a,b and c.If the H.C.F of a a and b is 2 , b and c is 5, and a and c is 3 , find the three numbers.

The three numbers are: a = 6, b = 10, c = 15

The factors of a: 6: 1, 2, 3, 6
The factors of b: 10: 1, 2, 5, 10
The factors of c: 15: 1, 3, 5, 15
HCF of a and b (6 and 10) = 2
HCF of b and c (10 and 15) = 5
HCF of a and c (6 and 15) = 3


4. Write down two numbers with H.C.F. as a multiple of 4 and L.C.M. as 240.

Two numbers are: 20, 48

The factors of 20 = 1, 2,4, 5, 10, 20
The factors of 48 = 1,2,3,4,6,8,12,16,24
The HCF of 20 and 48 = 4

The multiple of 20 = 20,40,60,80,100,120,140,160,180,200,220,240
The multiple of 48 = 48, 96,,144,192, 240
The LCM of 20 and 48 = 240


5. Write down two numbers such that their H.C.F is 1 and their L.C.M.is
(a) an even number

two numbers are 3, 4

Factors of 3: 1, 3
Factors of 4:1, 2, 4
HCF of 3 and 4 = 1

Multiple of 3: 3,6,9,12
Multiple of 4: 4,8, 12
LCM of 3 and 4 = 12 (the LCM is an even number)

(b) an odd number

Two numbers are 3, 5

Factors of 3: 1, 3
Factors of 5:1, 5
HCF of 3 and 5 = 1

Multiple of 3: 3,6,9,12,15
Multiple of 5: 5,10,15
LCM of 3 and 5 = 15 (the LCM is an odd number)