SOLUTION: a number consisting of two digits. The digit in the tens place of is twice the digit in the unit place.show that the number is the sum of the digits.

Sorry, that is not true!  Suppose the digit in the tens place is 4 and the
digit in the units place is 2. Then the digit in the tens place is twice the
digit in the unit place.  That makes the number be 42.  However the sum of
the digits is 6, not 42. 

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I have a hunch you meant this problem instead:

A number consists of two digits. The digit in the tens place is twice the digit
in the unit place.show that the number is seven times the sum of
the digits.


Let u = the units or ones digit

>>...The digit in the tens place of is twice the digit in the unit place...<<

Therefore the tens digit is 2u

The number is 10 times the tens digit plus the ones digit.

The number = 10(2u) + u = 20u + u = 21u

The sum of the digits is 2u + u = 3u

21u = 7·3u

So this is true.

Edwin