d + 5 >= 17
We can subtract 5 from each side:
d >= 12
To get 20 on the right, we have to add 8 to each side (20-12=8).
d + 8 >= 20
To get d - 5 from the first expression (d + 5 >= 17) we have to subtract ten from each side (5-10=-5) and we get:
d - 5 >= 7
