Measurement and Errors

A micrometer screw gauge has a negative error of 7 divisions. While measuring the diameter of a wire the reading on main scale is 2 divisions and $$78^{th}$$ circular scale division, coincides with base line. The number of divisions on main scale are 10 to a centimetre and the circular scale has 100 divisions.
N divisions on the main scale of vernier callipers coincide with (N+1) divisions on the vernier scale. If each divisions on the main scale of a units, determine the least count of instrument.
Subtract $$2.5 \times {10}^{4}$$ from $$3.9 \times {10}^{5}$$ and give the answer to correct number of significant figures.
Two rods have lengths measured as $$\left( 1.8\pm 0.2 \right)m$$ and $$\left( 2.3\pm 0.1 \right)m$$. Calculate their combined length with error limits.
You have a bag of cotton and an iron bar, each indicating a mass of $$100kg$$ when measured on a weighing machine. In reality, one is heavier than the other. Can you say which one is heavier and why?