In a week, her birds eat a total of 100 grams + 50 grams = <<100+50=150>>150 grams of seed each week.
Leah has a total of 3 + 5 = <<3+5=8>>8 boxes of birdseed.
The boxes of birdseed contain a total of 8 * 225 grams = <<8*225=1800>>1800 grams.
This is enough birdseed to feed her pets for 1800 / 150 = <<1800/150=12>>12 weeks.
#### 12