Answer:
average mass of 4 boys=45
or, average mass of 4 boys=sum of weight of 4 boys/4
or,45=total weight/4
therefore, total weight=180kgs
when a new boy joins average mass becomes 40,so,
average mass of 5 boys=40
or,avaerage mass of 5 boys=sum of weights of all 5 boys/5
or,40=(180+x)/5[since the total weight of 4 boys was 180 kgs from the previous calculation and considering 'x' as the weight of the new boy]
or,200=180+x
so,x=200-180
x=20
therefore the mass of the fifth boy is 20kg
