Yi's average for her first 3 tests is 88. what musit she score on her last test to bring her average up to 90

Her average now is 88.
She needs to bring it up to a 90.

Let's start with if she gets a 90.
88 + 90 = 178
178/2 = 89
A 90 would only get her an average of 89, so you need to go higher.

88 + 91 = 179
179/2 = 89.5

88 + 92 = 180
180/2 = 90

She must get at least a 92 to get an average of 90.

A=3
B=4
C=5
Substitute 3 in for 'a' in the equation, and 5 in for 'c' in the equation to get 17.