Question

the sum of two numbers is 90 their diffrence is 12 whats the number

Answer 1

You could easily do this by splitting 90 in half and just shifting the numbers until you have a number that has 12 more than the other

Answer 2

Yeah you could use guess-and-check as AngeI said or form a system of linear equations

Let \(\text{x}\) and \(\text{y}\) represent the numbers

\(\text{(1)...x+y=90}\)
\(\text{(2)...x-y=12}\)

Then use elimination and solve for both numbers x and y