On Tuesday, Billy ate 2 * 2 = <<2*2=4>>4 apples.
On Friday, he ate 2 * 0.5 = <<2*0.5=1>>1 apple.
On Thursday, he ate 4 * 1 = <<4*1=4>>4 apples.
So for Monday, Tuesday, Thursday, and Friday, Billy ate a total of 2 + 4 + 4 + 1 = <<2+4+4+1=11>>11 apples.
This means that on Wednesday, he must have eaten 20 – 11 = <<20-11=9>>9 apples.
#### 9