diff --git a/src/hubbleds/components/doppler_slideshow/SlideshowDopplerCalc5.vue b/src/hubbleds/components/doppler_slideshow/SlideshowDopplerCalc5.vue
index 3a160241c..dca95a051 100644
--- a/src/hubbleds/components/doppler_slideshow/SlideshowDopplerCalc5.vue
+++ b/src/hubbleds/components/doppler_slideshow/SlideshowDopplerCalc5.vue
@@ -50,7 +50,7 @@
         >
           <v-lazy>
             <v-card-text
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
                 class="pt-8"
             >
               <p>
@@ -162,7 +162,7 @@
         >
           <v-lazy>
             <v-card-text
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
                 class="pt-8"
             >
               <v-card
@@ -277,7 +277,7 @@
         >
           <v-lazy>
             <v-card-text
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
                 class="pt-8"
             >
               <v-card
@@ -395,7 +395,7 @@
         >
           <v-lazy>
             <v-card-text
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
                 class="pt-8"
             >
               <v-card
@@ -522,7 +522,7 @@
         >
           <v-lazy>
             <v-card-text
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
                 class="pt-8"
             >
               <p>
@@ -656,11 +656,12 @@
 
         <v-window-item :value="5"
                        class="no-transition"
+                       ref="last_step"
         >
           <v-lazy>
             <v-card-text
                 class="pt-8"
-                v-intersect="typesetMathJax"
+                v-intersect="mathJaxObserver"
             >
               <v-card
                   class="past_block pa-3"
@@ -845,10 +846,10 @@
             color="accent"
             elevation="2"
             @click="() => {
-              validateLightSpeed(['speed_light']) ? set_step(step + 1) : null;
               if (validateLightSpeed(['speed_light'])) {
                 set_student_c(parseAnswer(['speed_light']));
                 storeStudentVel(parseAnswer(['speed_light']), [lambda_obs, lambda_rest]);
+                set_step(step + 1);
               }
           }"
         >
@@ -927,14 +928,30 @@ mjx-mpadded {
 <script>
 export default {
   methods: {
-    typesetMathJax(entries, _observer, intersecting) {
+    mathJaxObserver(entries, _observer, intersecting) {
       if (intersecting) {
         this.$nextTick(() => {
-          MathJax.typesetPromise(entries.map(entry => entry.target));
+          this.refreshMathJax(entries.map(entry => entry.target));
         });
       }
     },
 
+    removeMathJax(containers) {
+      containers.forEach(container => container.querySelectorAll("mjx-container").forEach(el => container.remove(el)));
+      MathJax.typesetClear(containers);
+    },
+
+    refreshMathJax(containers) {
+      const containersToReset = containers.filter(this.containsMathJax);
+      this.removeMathJax(containersToReset);
+      this.$nextTick(() => MathJax.typesetPromise(containersToReset));
+    },
+
+    containsMathJax(container) {
+      const mathJaxOpeningDelimiters = [ "$$", "\\(", "\\[" ];
+      return mathJaxOpeningDelimiters.some(delim => container.innerHTML.includes(delim));
+    },
+
     getValue(inputID) {
       const input = document.getElementById(inputID);
       if (!input) {
diff --git a/src/hubbleds/pages/01-spectra-&-velocity/guidelines/GuidelineDopplerCalc4.vue b/src/hubbleds/pages/01-spectra-&-velocity/guidelines/GuidelineDopplerCalc4.vue
index 09d6d3a42..649a1080e 100644
--- a/src/hubbleds/pages/01-spectra-&-velocity/guidelines/GuidelineDopplerCalc4.vue
+++ b/src/hubbleds/pages/01-spectra-&-velocity/guidelines/GuidelineDopplerCalc4.vue
@@ -198,7 +198,7 @@ export default {
         const value = this.parseAnswer(id);
         console.log(id, index, value, expectedAnswers[index], value && value === expectedAnswers[index]);
         // this.failed_validation_4_callback((!(value && value === expectedAnswers[index])));
-        return value && value === expectedAnswers[index];
+        return value != null && value === expectedAnswers[index];
       });
     }
   },