Their sum is 21 and their difference is 7

Find the value of two numbers, when their sum is 21 and their difference is 7.

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x = one number
y = other number

x + y = 21  {their sum is 21}
x - y = 7 {their difference is 7}
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2x = 28  {added the two equations}
x = 14   {divided each side by 2}

x + y = 21  {first equation}
14 + y = 21  {substituted 14, in for x, into first equation}
y = 7  {subtracted 14 from each side}

the two numbers are 14 and 7

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