Question 1140893
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{{{4*70<=(82+88+87+x)<=4*79}}}
{{{280<=257+x<=316}}}
{{{23<=x<=59}}}<br>
ANSWER: [23,59]<br>
Note the stated condition for getting a C is probably not correct.  The statement says a C grade means an average in the interval [70,79]; probably the correct interval for getting a C grade is [70,80).<br>
That will of course change the answer to the problem slightly.