A number is xy (like 15 where x is 1 and y is 5). x and y are the digits with y in the ones place and x in the tens place.
The problem indicates that the tens digit is 8, so the number is 8y where 8+y = 15.
Subtract 8 from both sides to find y=7.
The number is 87.
In the problem you indicated that 8 was in the "tenths" place. If it really is tenths place, and not tens place, then the number is x.y where y (the tenths place) is 8.
The other number is still x=15-8=7, but the number is 7.8.
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