fix java-cast-error

src/cljc/lernmeister/components/exercise_types/calculation/check.cljc

 (ns lernmeister.components.exercise-types.calculation.check
-  (:require [lernmeister.components.exercise-types.calculation.migrate-answer
+  (:require [clojure.tools.reader.edn :as edn]
+            [lernmeister.components.exercise-types.calculation.migrate-answer
              :refer
              [answer-scheme-version migrate-answer]]
             [lernmeister.components.exercise-types.check :as e-check]
@@ -19,7 +20,8 @@
 
 (defn check-calculation [id ans-obj ex-obj res-map with-oom with-units]
   (let [phys-val-check-res (check-phys-vals ex-obj ans-obj :with-oom with-oom :with-units with-units)
-        is-correct? (tolerable-error? (:calculation-error phys-val-check-res) (int (:error-relative ex-obj)))]
+        is-correct? (tolerable-error? (:calculation-error phys-val-check-res)
+                                      (edn/read-string (:error-relative ex-obj)))]
     (assoc res-map id {:correct is-correct?
                        :points (if is-correct? (:correct-points ex-obj) (:incorrect-points ex-obj))
                        :points-max (:correct-points ex-obj)})))