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
#### 1000