diff --git a/miplearn/problems/stab.py b/miplearn/problems/stab.py
index 6da07df..8bfd504 100644
--- a/miplearn/problems/stab.py
+++ b/miplearn/problems/stab.py
@@ -11,18 +11,24 @@ from scipy.stats import uniform, randint, bernoulli
 from scipy.stats.distributions import rv_frozen
 
 
-class MaxWeightStableSetChallengeA:
-    def __init__(self):
+class ChallengeA:
+    def __init__(self,
+                 seed=42,
+                 n_training_instances=300,
+                 n_test_instances=50,
+                ):
+        
+        np.random.seed(seed)
         self.generator = MaxWeightStableSetGenerator(w=uniform(loc=100., scale=50.),
                                                      n=randint(low=200, high=201),
                                                      p=uniform(loc=0.05, scale=0.0),
                                                      fix_graph=True)
-    
-    def get_training_instances(self):
-        return self.generator.generate(300)
-    
-    def get_test_instances(self):
-        return self.generator.generate(50)
+        
+        np.random.seed(seed + 1)
+        self.training_instances = self.generator.generate(n_training_instances)
+        
+        np.random.seed(seed + 2)
+        self.test_instances = self.generator.generate(n_test_instances)
 
 
 class MaxWeightStableSetGenerator: