Explaination :
Let us assume the unknown two numbers as an variable. Here we are assuming it as a and b.
★ According to the question,
Sum of those assumed two numbers is 82.5.
a + b = 82.5
Also it is given that difference of both the two numbers is 12.9.
a - b = 12.9
From the given information we got two equations,
a + b = 82.5 (Equation No 1)
a - b = 12.9 (Equation No 2)
So we would solve them and get the value of a and b.
★ From Equation No 2 :
a = 12.9 + b
★ In Equation No 1 :
Substituting the value of a which we got above here,
(12.9 + b) + b = 82.5
12.9 + b + b = 82.5
12.9 + 2b = 82.5
2b = 82.5 - 12.9
2b = 69.6
b = 69.6 / 2
b = 34.8
★ Therefore,
★ Finding out value of a :
a = 12.9 + b
a = 12.9 + 34.8
a = 47.7