Answer: 54
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Work Shown:
T = tens digit
U = units digit (aka ones digit)
A number like 27 is really 20+7 = 2*10 + 7*1 = 10*2 + 1*7. We have 2 in the tens digit and 7 in the units digit. So 27 can be written in the form 10T + U where T = 2 and U = 7. Reversing the digits gives 72, so T = 7 and U = 2 now. Clearly the difference between the digits 7 and 2 is not 1, so 27 or 72 is not the answer (as it's just an example).
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Let T be larger than U. This doesn't work if T = U.
Because T is larger, saying "The difference between the digits is 1" means T - U = 1. We can isolate T to get T = U+1. We'll use this later.
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If T > U, then the original number 10T+U reverses to the new number 10U+T and it becomes smaller. We are told that it becomes 5/6 of what it used to be.
So,
new number = (5/6)*(old number)
10U + T = (5/6)(10T + U)
6(10U + T) = 5(10T + U)
60U + 6T = 50T + 5U
60U + 6(U+1) = 50(U+1) + 5U ... plug in T = U+1
60U + 6U + 6 = 50U + 50 + 5U
66U + 6 = 55U + 50
66U - 55U = 50-6
11U = 44
U = 44/11
U = 4 is the units digit of the original number
T = U+1
T = 4+1
T = 5 is the tens digit of the original number
The original number is therefore 10T + U = 10*5+4 = 54.
We see the difference in their digits is T-U = 5-4 = 1
The reverse of 54 is 45. The number 45 is 5/6 of 54
45 = (5/6)*54