Question 315550
Let S be the score from the second test, T the score from the third.
{{{ (63+T+S)/3=78}}}
{{{ 63+T+S=234}}}
.
.
{{{ T=S+9}}}
{{{ 63+(S+9)+S=234}}}
{{{ 2S+72=234}}}
{{{ 2S=162}}}
{{{ highlight(S=81)}}}
Then from above,
{{{ T=S+9}}}
{{{ T=81+9}}}
{{{ highlight(T=90) }}}