Consider the question. Mrs. Snyder used to spend 40% of her monthly income on rent and utilities. Her salary was recently increased by $600 so now her rent and utilities only amount to 25% of her monthly income. How much was her previous monthly income?
 If the answer is '1000'; explain the reasoning:
Let her previous monthly income be p. The cost of her rent and utilities was 40% of p which is (40 / 100) * p = 2p / 5. Her income was increased by $600 so it is now p + $600. The cost of her rent and utilities now amount to 25% of (p + $600) which is (25 / 100) * (p + $600) = (p + $600) / 4. Equating both expressions for cost of rent and utilities: 2p / 5 = (p + $600) / 4. Multiplying both sides of the equation by 20 gives 8p = 5p + $3000. Subtracting 5p from both sides gives: 3p = $3000. Dividing both sides by 3 gives p = $1000.