The total for an average to be 87 would have be 87 x the number of tests or 87 x 4 = 348.
Let's see the total for her previous 3: 82 + 86 + 91 = 259
The 4th one needs to take her from 259 to 348.
348 - 259 = 89. she has 89 'to go" to get that 87 average.
She requires an 89 to achieve an 87 test average.