Question 1006157
Using definition of average:
(1) {{{ ( a+ b ) / 2 = 9 }}}
(2) {{{ ( b + c ) / 2 = 6 }}}
(3) {{{ ( c + d ) / 2 = 15 }}}
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Multiply both sides by {{{ 2 }}}
(1) {{{ a + b = 18 }}}
(2) {{{ b + c = 12 }}}
(3) {{{ c + d = 30 }}}
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Subtract (2) from (1)
(1) {{{ a + b = 18 }}}
(2) {{{ -c - b = -12 }}}
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{{{ a - c = 6 }}}
Add (3) to this result
{{{ a - c = 6 }}}
{{{ d + c = 30 }}}
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{{{ a + d = 36 }}}
Now, to get the definition of 
an average, divide both sides by {{{ 2 }}}
{{{ ( a + d ) / 2 =18 }}}
The average of a and d is {{{ 18 }}}