A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations as :

**S. No.** |
**Distance of flame from the lens (cm)** |
**Distance of the screen from the lens (cm)** |

(i) |
60 |
20 |

(ii) |
40 |
24 |

(iii) |
30 |
30 |

(iv) |
24 |
40 |

(v) |
15 |
70 |

(a) From the above table, find the focal length of lens without using lens formula.

(b) Which set of observations is incorrect and why ?

(c) In which case, the size of the object and image will be same ? Give reason for your answer.