Question:
Find all pairs of consecutive even positive integers, both of which are larger than 5 such that their sum is less than 23.
Solution:
Let x be the smaller of the two consecutive even positive integers. Then, the other integer is x + 2.
Since both the integers are larger than 5,
$x>5 \ldots$ (1)
Also, the sum of the two integers is less than 23 .
$x+(x+2)<23$
$\Rightarrow 2 x+2<23$
$\Rightarrow 2 x<23-2$
$\Rightarrow 2 x<21$
$\Rightarrow x<\frac{21}{2}$
$\Rightarrow x<10.5$ $\ldots$ (2)
From (1) and (2), we obtain $5
Since $x$ is an even number, $x$ can take the values, 6,8 , and 10 .
Thus, the required possible pairs are $(6,8),(8,10)$, and $(10,12)$.