From its total income, A sales company spent Rs.20000 for advertising, half of the remainder on commissions and had Rs.6000 left. What was its total income?

You can write an equation expressing the information you know.
  (total income) - 20,000 - (1/2)*((total income) - 20,000) = 6,000
Solving for total income, we have
  (total income) - 20,000 - (1/2)*(total income) + 10,000 = 6,000
  (1/2)*(total income) = 16,000    (collect terms, add 10,000 to both sides)
  (total income) = 32,000    (multiply by 2)

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You can recognize that "the other half" of the amount not spent on commissions is the Rs 6000 that remains. This means commissions were 6000 and the total is
  20,000 + 6,000 + 6,000 = 32,000