Question 1201921
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{{{ 70 <= (76+94+77+x)/4 <= 79 }}}

Multiply by 4 and partly simplify middle term:
{{{ 280 <= 247 + x <= 316 }}}

Subtract 247 from all three sides:
{{{ 33 <= x <= 69 }}}

Mark needs to score in the range 33 to 69, inclusive, to obtain a C.