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Calculation puzzle image

I need help to work out how to solve this puzzle. The puzzle is very self-explanatory...

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3
  • $\begingroup$ What is the source of this puzzle? $\endgroup$ Jun 28 at 5:45
  • $\begingroup$ no idea, a fellow mate asked me to solve it $\endgroup$
    – Hmmwell
    Jun 28 at 6:04
  • 1
    $\begingroup$ 4 variables and 4 equations, wolframalpha gives a solution $9/10, 21/10,59/10,10$ $\endgroup$
    – z100
    Jun 28 at 6:24
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This is a system of 4 equations with 4 variables:

$a+b=8$
$a*c=21$
$b-d=5$
$c*d=9$

From 2nd and 4th equation:

$a=21/c$
$d=9/c$

For easier writing let's put $1/c=x$ and insert $a$ and $d$ into 1st and 3rd equation:

$21x+b=8$
$b-9x=5$
so $30x = 3$ or $x=1/10$

From here it is easy:

$c=10$, $a=21/10$, $d=9/10$ and $b=59/10$

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