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the sum of two numbers15. the difference of these same numbers is 7

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1 Answer

Let n = one of the number and m = the other number.  You have two unknowns (m and n), so you need two equations to solve for the unknowns.
  1. The sum of two numbers is 15:  n + m = 15
  2. The difference of the same numbers is 7:  n - m = 7   (Therefore n = 7 + m)
 
Substitute n = 7 + m from the second equation in place of n in the first equation:
 
n + m = 15              [Equation 1]
 
7 + m + m = 15       [Substituted 7+m in place of n]
 
Solve for m.
 
Once you have m, solve for n using n = 7 + m from equation 2.