The reason why it's <=:
In the problem, she draws at least $20 per week. She could take out more than $20.
Therefore, at a certain time, if she takes out more than $20, what she has in the bank will be, B<750-20w. (750 is the principal amount she has)
If she only takes out $20 each time, she'll have, B=750-20w, in the bank.
Hence the inequality, B<=750-20w.