diff --git a/class/Tasks/NumberConversionTask.php b/class/Tasks/NumberConversionTask.php
index f7404b8..a376651 100644
--- a/class/Tasks/NumberConversionTask.php
+++ b/class/Tasks/NumberConversionTask.php
@@ -76,20 +76,31 @@ class NumberConversionTask extends OpenEndedTask
             $this->src_rep = "u";
         }
 
+        // switch between source and destination constrains
+        if ($this->hasFlag("sourceconstrain")) {
+            $base = $this->src_base;
+            $n_digits = $this->src_n_digits;
+            $rep = $this->src_rep;
+        } else {
+            $base = $this->dst_base;
+            $n_digits = $this->dst_n_digits;
+            $rep = $this->dst_rep;
+        }
+
         // specify the range
         $max = 1;
         $min = 0;
-        switch ($this->dst_rep) {
+        switch ($rep) {
             case "u":
-                $max = pow($this->dst_base, $this->dst_n_digits) - 1;
+                $max = pow($base, $n_digits) - 1;
                 $min = 0;
                 break;
             case "s":
-                $max = pow($this->dst_base, $this->dst_n_digits) - 1;
+                $max = pow($base, $n_digits) - 1;
                 $min = -$max;
                 break;
             case "c":
-                $max = pow($this->dst_base, $this->dst_n_digits - 1) - 1;
+                $max = pow($base, $n_digits - 1) - 1;
                 $min = -($max + 1);
                 break;
         }
@@ -106,11 +117,17 @@ class NumberConversionTask extends OpenEndedTask
     {
         $mark = 0.0;
         $pa = strtolower($this->player_answer);
-        if ($this->hasFlag("acceptwithoutleadingzeros")) {
-            $mark = (ltrim($pa, " 0") === ltrim($this->correct_answer, "0")) ? $this->getMaxMark() : 0.0;
-        } else {
-            $mark = (trim($pa) === trim($this->correct_answer)) ? $this->getMaxMark() : 0.0;
+        $ca = $this->correct_answer;
+        if (($ca[0] === "-") && ($pa[0] !== "-")) {
+            goto setmark;
         }
+        if ($this->hasFlag("acceptwithoutleadingzeros")) {
+            $mark = (ltrim($pa, "+- 0") === ltrim($ca, "-0")) ? $this->getMaxMark() : 0.0;
+        } else {
+            $mark = (trim($pa) === trim($ca)) ? $this->getMaxMark() : 0.0;
+        }
+
+        setmark:
         $this->setMark($mark);
     }
 }
\ No newline at end of file