Joe’s dad roasts 21/3 = <<21/3=7>>7 of his marshmallows.
Joe has 21*4 = <<21*4=84>>84 marshmallows.
Joe roasts 84/2 = <<84/2=42>>42 of his marshmallows.
Joe and his dad roasted 7+42 = <<7+42=49>>49 marshmallows in total.
#### 49