We were told that Mai made $192 for 12 hours of work. This means that the amount that she made per hour is
192/12 = $16
Given that her constant rate is $16 per hour,
let x = the number of hours would she have to work to make $128. Then, we have the following equations
1 = 16
x = 128
By crossmultiplying, we have
16x = 128
x = 128/16
x = 8
She has to work for 8 hours