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2014-10-04T17:42:38+08:00

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So first it is a 2 digit number so think of numbers from 10-99 then if u add them its 15 and if multiplied it is 56

let x=first digit
let y=second digit

x+y=15
then the other one is
xy=15

so first lets start with xy=15
xy=15
divide both sides by y
x=15/y

x+y=15
15/y+y=15
15/y+y^2/y=15
(15+y^2)/y=15
Multiply both sides by y
15+y^2=15y
y^2-15y+15=0
(y-8)(y-7)=0
y=7 y=8

Substitute it from first equation
x+y=15
x+7=15
x=8

x+y=15
x+8=15
x=7

There for it must be vice versa so it should be 78 where 7 is the x and 8 is the y and 87 where x is 8 and 7 is y

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2014-10-05T10:52:52+08:00

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8+7=15
8x7=56,
therefore: the two-digit defined is 8 and 7;
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