div.invisible{
		
	}
	
	body{
		background-image: url('background.jpg');
	}
	
	#equation{
		font-size: 100px;
	}
	
	#correct{
		color: #44c785;
		font-size: 25px;
	}
	
	#incorrect{
		color: #ffa600;
		font-size: 25px;
	}
	
	#questions{
		text-align: center;
		font-size: 25px;
	}
	
	#end{
		text-align: center;
	}
	
	#error{
		text-align: center;
		font-size: 25px;
		color: #ffa600;
	}
	
	#math{
		text-align: center;
	}
	
	#numCorrect{
		font-size: 25px;
	}
	
	
	.flip-card {
	  background-color: transparent;
	  width: 300px;
	  height: 300px;
	  perspective: 1000px;
	  display: inline-block;
	  
	}

	.flip-card-inner {
	  position: absolute;
	  width: 100%;
	  height: 100%;
	  text-align: center;
	  transition: transform 0.9s;
	  transform-style: preserve-3d;
	  box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2);
	}
	
	.flip-card-inner.flipped{
		transform: rotateX(180deg);
	}

	.flip-card-front, .flip-card-back {
	  position: absolute;
	  width: 100%;
	  height: 100%;
	  
	  -webkit-backface-visibility: hidden;
	  backface-visibility: hidden;
	  overflow: auto;
	}

	.flip-card-front {
	  background-color: #2980b9;
	  color: black;
	  line-height: 250px;
	}

	.flip-card-back {
	  background-color: #2980b9;
	  color: black;
	  font-size: 100px;
	  transform: rotateX(180deg);
	}