# Question 21

**Express the following numbers in standard form**

**(i) 0.0000000000085 (iv) 0.00000000837**

**(ii) 0.00000000000942 (v) 31860000000**

**Solution:**

(i) 0.0000000000085

0.0000000000085 = 8.5 Ã— 10 ^{-12}

(ii) 0.00000000000942

0.00000000000942 = 9.42 Ã— 10 ^{-2}

(iii) 6020000000000000

6020000000000000 = 6.02 Ã— 10^{15}

(iv) 0.00000000837

0.00000000837 = 8.37 Ã— 10^{-9}

(v) 31860000000

31860000000 = 3.186 Ã— 10^{10}

# Question 25

**Express the number appearing in the following statements in standard form:**

**(ii) Charge of an electron is 0.000,000,000,000,000,000,16 coulomb**

**(iii) Size of a bacteria is 0.0000005m**

**(iv) Size of a plant cell is 0.00001275m**

**(v) Thickness of a thick paper is 0.07 mm.**

**Solution:**

(i) 1micron is equal to

(ii) Charge of an electron is 0.000,000,000,000,000,000,16 coulomb

Charge of an electron =

(iii) Size of a bacteria is 0.0000005m

Size of a bacteria =

(iv) Size of a plant cell is 0.00001275m

Size of a plant cell =

(v) Thickness of a thick paper is 0.07 mm

Thickness of a thick paper .

# Question 26

**In a stock there are 5 books each of thickness 20mm and 5 paper sheets each of thickness 0.016 mm. What is the total thickness of the stock?**

**Solution:**

Thickness of one book = 20 mm

Thickness of 5 books = 20 Ã— 5 = 100 mm ----(1)

Thickness of one paper = 0.016

Thickness of 5 papers = 5 Ã— 0.016 = 0.08 mm ----(2)

(1) + (2) = 100 + 0.08 = 100.08 or 1.0008 Ã— 10^{2}.