diff --git a/aoc2020/lib/aoc2020.dart b/aoc2020/lib/aoc2020.dart
index 3654ed1..af7a6a4 100644
--- a/aoc2020/lib/aoc2020.dart
+++ b/aoc2020/lib/aoc2020.dart
@@ -10,6 +10,7 @@ import 'package:aoc2020/aoc20201209.dart';
 import 'package:aoc2020/aoc20201210.dart';
 import 'package:aoc2020/aoc20201211.dart';
 import 'package:aoc2020/aoc20201212.dart';
+import 'package:aoc2020/aoc20201213.dart';
 import 'package:aoc2020/aocbase.dart';
 import 'dart:mirrors';
 
@@ -32,6 +33,7 @@ class aoc2020 {
     AOC20201210(),
     AOC20201211(),
     AOC20201212(),
+    AOC20201213(),
   ];
 
   static void list() async {
diff --git a/aoc2020/lib/aoc20201213.dart b/aoc2020/lib/aoc20201213.dart
index 161888f..55710ad 100644
--- a/aoc2020/lib/aoc20201213.dart
+++ b/aoc2020/lib/aoc20201213.dart
@@ -1,20 +1,77 @@
+import 'dart:math';
 import 'package:aoc2020/aocbase.dart';
 import 'package:aoc2020/model/readdata.dart';
 
 class AOC20201213 extends AOCBase {
   var mylist = <String>[];
+  int get arrival => int.parse(mylist.first);
+  var busId = <int>[];
+  var busIdSec2 = <int>[];
 
   void init(bool test) async {
     mylist = await ReadData.readFile<String>(classString, test: test);
+    busId = mylist.last.split(',').where((id) => id != 'x').toList().map((id) => int.parse(id)).toList();
+    busIdSec2 = mylist.last.split(',').map((id) => id == 'x' ? 1 : int.parse(id)).toList();
   }
 
   @override
   Future<void> a({bool test}) async {
     await init(test);
+    var _mods = busId.map((id) => id - (arrival % id)).toList();
+    var _minTime = _mods.reduce(min);
+    var myBusId = busId[_mods.indexOf(_minTime)];
+    answerA = myBusId * _minTime;
   }
 
   @override
   Future<void> b({bool test}) async {
     await init(test);
+    var maxTime = busIdSec2.reduce(max);
+    var indexOfMax = busIdSec2.indexOf(maxTime);
+    var offsets = <int>[];
+    for (var index = 0; index < busIdSec2.length; index++) {
+      if (busIdSec2[index] != 1) offsets.add(index - indexOfMax);
+    }
+    // var mapOfBusIds =  = busId.asMap();
+    var mapOfBusIdtoOffsets = <int, int>{};
+    var mapOfIndexToBusId = <int, int>{};
+    for (var i = 0; i < busId.length; i++) {
+      mapOfBusIdtoOffsets[busId[i]] = offsets[i];
+      mapOfIndexToBusId[i] = busId[i];
+    }
+    busId.sort((b, a) => a.compareTo(b));
+
+    // void printSched(int time, int increment, int currIndex) {
+    //   stdout.write('$time [$increment:$currIndex]');
+    //   for (var index = 0; index < busId.length; index++) {
+    //     var busid = mapOfIndexToBusId[index];
+    //     var str = (time + mapOfBusIdtoOffsets[busid]) % busid == 0 ? 'D' : '.';
+    //     stdout.write('    $str');
+    //   }
+    //   print(' ');
+    // }
+
+    int calcFor({int number, int inputTime, int increment}) {
+      var time = inputTime;
+      var notdone = true;
+      do {
+        var fail = false;
+        time += increment;
+        for (var index = 0; index < number && !fail; index++) {
+          var offset = mapOfBusIdtoOffsets[busId[index]];
+          fail = (time + offset) % busId[index] != 0;
+        }
+        notdone = fail;
+      } while (notdone && time > 0);
+      return time;
+    }
+
+    var time1 = calcFor(number: 4, inputTime: 0, increment: maxTime);
+    var time2 = calcFor(number: 4, inputTime: time1, increment: maxTime);
+    var diffTime = time2 - time1;
+    print('$time2 - $time1 = ${diffTime}');
+
+    var time = calcFor(number: busId.length, inputTime: time2, increment: diffTime);
+    answerB = time - indexOfMax;
   }
 }
diff --git a/aoc2020/lib/data/aoc20201213.data b/aoc2020/lib/data/aoc20201213.data
new file mode 100644
index 0000000..11e789e
--- /dev/null
+++ b/aoc2020/lib/data/aoc20201213.data
@@ -0,0 +1,2 @@
+1000299
+41,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,971,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,13,x,x,x,x,23,x,x,x,x,x,29,x,487,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,19
diff --git a/aoc2020/lib/data/aoc20201213test.data b/aoc2020/lib/data/aoc20201213test.data
new file mode 100644
index 0000000..9d971e3
--- /dev/null
+++ b/aoc2020/lib/data/aoc20201213test.data
@@ -0,0 +1,7 @@
+939
+17,x,13,19
+67,7,59,61
+67,x,7,59,61
+67,7,x,59,61
+1789,37,47,1889
+7,13,x,x,59,x,31,19