Total money I have = $350.
Newspaper normal charges = $100 per add.
Discount of each add = 30% of 100 = 0.30 * 100 = $30 .
Cost of each add after discount = 100-30 = $70.
Let us assume I can buy = x adds.
Number of adds * cost of one add after discount = 350.
x * 70 = 350.
70x = 350.
Dividing both sides by 70, we get
70x/70 = 350/70
x = 5.
Therefore, I can buy 5 addes.