Question

what should be subtracted from thrice the rational number -8/3 to get 5/2

Answer 1

3*-8/3=-8        -8-x=5/2            -8-5/2=x              -16/2-5/2=x                 -21/2=x

Answer 2

let the number be x
3*-8/3-x=5/2
-8-x=5/2
x=-8-5/2
x=(-16-5)/2
x=-21/2