Let's represent the cost for each company as a system of equations:
Let x be the number of balloons ordered and y be the total cost.
For Balloon Express:
2x + 10 = y
For Balloon Mania:
1.5x + 20 = y
We know that the O'Donnells plan to order 30 balloons, so x = 30.
Now we can plug in x = 30 into both equations to find the total cost for each company:
For Balloon Express:
2(30) + 10 = 60 + 10 = 70
For Balloon Mania:
1.5(30) + 20 = 45 + 20 = 65
Therefore, the O'Donnells should choose Balloon Mania as it will cost them $65, saving $5 compared to Balloon Express.