The average mass of 4 boys is 45kg. When a fifth boy joins them the average mass changes to 40kg. What is the mass of the fifth boy?​

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

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