Answer:
Wei can purchase 300 balloons.
The model used for the problem is
.
Explanation:
Let the number of white balloons = x
The cost of blue balloons and white balloons are 60 cent = $0.6 and 30 cents = $0.3 respectively.
Wei purchased 100 blue balloons and have $150 to buy both balloons.
So, we get the relation,

i.e.

i.e.

i.e.

Thus, Wei can purchase 300 balloons.
So, the model used for the problem is
.