An average of 89 people attended four performances of a play over the weekend. on friday night, 87 people attended and on saturday night, 95 people attended. on sunday, there was an afternoon performance with 83 attendees and another performance at night. how many people attended the sunday night performance? a performance on monday changed the average attendance to 90. how many people attended the monday performance?

91 people attended Sunday night.  94 people attended Monday.

The average is found by adding together all of the data points and dividing by the number of data points.  For the Sunday night performance, we have
89 = (87+95+83+x)/4

Multiply both sides by 4:
89(4) = ((87+95+83+x)/4)*4
356 = 265+x

Subtract 265 from both sides:
356-265 = 265+x-265
91 = x

For the Monday performance:
90 = (87+95+83+91+x)/5

Multiply both sides by 5:
90(5) = ((87+95+83+91+x)/5)*5
450 = 356 + x

Subtract 356 from both sides:
450-356 = 356+x-356
94 = x