Half of Tanya's 20 erasers are red, so she has 20/2=<<20/2=10>>10 red erasers.
One-half as many erasers as Tanya has red erasers is 10/2=<<10/2=5>>5 red erasers.
Rachel's three less than one-half as many erasers as Tanya's red erasers, which is 5-3=2 erasers.
Hanna has twice as many erasers as Rachel, for a total of 2*2=<<2*2=4>>4 erasers.
#### 4