Question 1025833
Let {{{ x }}} = the grade he should get in the next test
In words, 
[ the average score] = [ total of all the scores ] / [ the number of test ]
----------------
The total for the 1st 6 tests is:
[ 6 tests ] x [ average score of 50 ]
{{{ 50*6 = 300 }}}
------------------
The total score for the 7 tests is:
{{{ 300 + x }}}
[ average score of 60 ] = [ total score for 7 tests ] / [ 7 tests ]
{{{ 60 = ( 300 + x ) / 7 }}}
{{{ 300 + x = 420 }}}
{{{ x = 120 }}}
He needs a score of 120 in the 7th test to average 60