Let the sum of ages of 19 students is x, so their average equation will be : x/19=50

therefore sum of ages of 19 students will be : x=50*19=950

After adding a student, there will be 20 students,

so the equation will be y/20=49.6 (y being the sum of ages of 20 students), so y= 992

Therefore weight of student added is 992-950=42kg