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

If the average mass of 4 boys is 45, that means that the sum of all buys divided by the quantity of them (4) is 45, so if we want to calculate the sum of their masses, we can solve this equation:

Then, we pass the 4 multiplying:

, so we know the four boys that were taken into account on thatt average sum 180 kg of mass

Now, if another boy joins with a mass of x, we can add it to the mass we gott and, as we know dividing that by 5 will give us 40, we solve the equation:

We pass the five multiplying:

Now we know that the fith of these boys has a mass of 20 kg

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step-by-step explanation: