Lisa's test grades are 79, 89, and 90. there will be one more test this year. if lisa wants her test average to be at least 88, what is the lowest grade she can get on her last test?

For this case, what we are going to do first is to assume that all the exams are worth the same percentage of the final grade.
 We have then that Lisa's average grade point equation is:
 
 Where,
 x: minimum note that lisa must obtain in the last exam.
 Clearing x we have:
 
 
 the lowest grade she can get on her last test is:
 x = 94