diff --git a/EasyLean/Basic.lean b/EasyLean/Basic.lean
index 305e1ca..1be7dee 100644
--- a/EasyLean/Basic.lean
+++ b/EasyLean/Basic.lean
@@ -1,12 +1,3 @@
-import Mathlib.Data.Real.Basic
-import Mathlib.Tactic.Linarith
+import EasyLean.Mathd.Algebra.MathdAlgebra513
 
-theorem mathd_algebra_513
-  (a b : ℝ)
-  (h₀ : 3 * a + 2 * b = 5)
-  (h₁ : a + b = 2) :
-  a = 1 ∧ b = 1 := by
-  sorry
-
-theorem eq_four : ∀ a b c d : Nat, a = b → a = d → a = c → c = b := by
-  sorry
+open EasyLean.Mathd.Algebra
diff --git a/EasyLean/Mathd/Algebra/MathdAlgebra513.lean b/EasyLean/Mathd/Algebra/MathdAlgebra513.lean
new file mode 100644
index 0000000..3810a0e
--- /dev/null
+++ b/EasyLean/Mathd/Algebra/MathdAlgebra513.lean
@@ -0,0 +1,13 @@
+import Mathlib.Data.Real.Basic
+import Mathlib.Tactic.Linarith
+
+namespace EasyLean.Mathd.Algebra
+
+/-- Solve the system `3a + 2b = 5`, `a + b = 2`. -/
+theorem mathd_algebra_513 (a b : ℝ) (h₀ : 3 * a + 2 * b = 5) (h₁ : a + b = 2) :
+    a = 1 ∧ b = 1 := by
+  constructor
+  · linarith [h₀, h₁]
+  · linarith [h₀, h₁]
+
+end EasyLean.Mathd.Algebra