Mary will inflate a total of 10 balloons/minute x 30 minutes = <<10*30=300>>300 balloons
Jess will inflate a total of 7 balloons/minute x 30 minutes = <<7*30=210>>210 balloons
Christina will inflate a total of 4 balloons/minute x 15 minutes = <<4*15=60>>60 balloons
Combining all balloons, they made 300 balloons + 210 balloons + 60 balloons = <<300+210+60=570>>570 balloons
#### 570