Question 1085222
digit one: x
digit two: y
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x+y = 15 (1)
if interchanged:
10y+x = 10x+y-9
10y+x-10x-y = -9
9y-9x = -9 divide all sides by 9:
y-x = -1 (2)
Add (1) and (2):
x+y = 15
+
-x+y = -1
__________
0+2y = 14
y = 7
the sum of the two digits = 15, so 15-7 = 8
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Check:
sum of the digits: 8+7 = 15
interchanged 9 less: 87-78 = 9 correct