Two houses

There are twice as many people living in the first house as in the second house. Altogether, there are 81 of them living in both. How many people live in each house?

Correct result:

a =  54
b =  27

Solution:

$$\begin{gathered} a=2b \\ a+b=81 \\ 2b+b=81 \\ b=81\mathrm{/}3=27 \\ a=2\cdot b=2\cdot 27=54 \end{gathered}$$

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