1. 15 × 3 .

2. One quarter of 48 .

    One quarter of 48 can be written as ¹/₄ × 48 = ⁴⁸/₄ = 12.
                                                             

3.
   

  Let x be the required number .
  24 + x = 100
  24 + x - 24 = 100 - 24 (subtracting both sides by 24 )
      

4. 10 % Rs 80.

    10 % Rs 80 = ¹⁰/₁₀₀ × 80
                                   = 8

5. 0.2 × 10 .

    2 × 10 = 20
        Since there is one decimal place in 0.2,put a decimal point one place from left of 20 .
       0.2 × 10 = 2.0
                    = 2 .

6. 50 + 39 .

7. Is 29 a prime number ?

    Yes 29 is a prime number because the factors of 29 are 1 and 29 itself .

8. 42 ÷ 7 .

   6 × 7 = 42
   42 ÷ 7 = 6 .

9. 80 - 27 .

10. Approximately , what is 39 × 11 ?

     10 is nearer to 11 .
      So 39 × 10 = 390
      Therefore 39 × 11 is approximately equal to 400 .