Question 1080008
let a = the 10's digit
let b = the units
:
10a + b = the number
:
Sum of two digits of a 2-digit number is 7.
a + b = 7
 If the digits are reversed, the new number so formed decrease by 27 .
10a + b = 10b + a + 27
10a - a = 10 - b + 27
9a = 9b + 27
simplify, divide by 9
a = b + 3
a - b = 3
 find the number 
Use elimination
a + b = 7
a - b = 3
-----------Addition eliminates b, find a
2a = 10
a = 5 is the first digit
you can do the rest