Question 829757
Two-digit number, {{{x*10+y*1}}}.
Expect the number {{{10x+y>20}}} and {{{10x+y<30}}}.
We must expect only Natural Numbers for x and y, so if graphing, these must be only in quadrant 1.


The other part of the description:
{{{highlight_green(3(x+y)=10x+y)}}}.
Simplify.
3x+3y=10x+y
3y=7x+y
2y=7x
{{{highlight_green(-7x+2y=0)}}}


Next plan is that we can plot that line.  Intercepts are (0,0) only.  The equation can be shown as {{{y=(7/2)x}}}; and because of the inequalities expected above, {{{y>-10x+20}}} and {{{y<-10x+30}}}.


The easiest path seems to be find all the Natural Number coordinate pairs on the line {{{y=(7/2)x}}} and stay at {{{y<10}}}.
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Best guess it the point, (2,7).
The number, {{{highlight_green(highlight(27))}}} ?
No other  natural number combination on that line for {{{y<10}}}.


CHECK THIS:
{{{3(2+7)=27}}}?
{{{3(9)=27}}} yes.