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Let them be x and y
Two numbers have a sum of 11
"Sum" means to stick a plus sign between them like this
x + y
So we're told that sum is 11. So you stick an equal sign after that
and then stick 11 after that. So you have
x + y = 11
>>...Two numbers have...a difference of 67...<<
"Difference" means to stick a minus sign between them like this
x - y
So we're told that difference is 67. So you stick an equal sign after that
and then stick 67 after that. So you have
x - y = 67
So you put those together as a system of equations
Answer: x=39, y=-28
When you add then, 39+(-28) that's 39-28 = 11
When you subtract them 38-(-28) that's 39+28=67.
Edwin