John climbs 3 staircases.  The first staircase has 20 steps.  The next has twice as many steps as the first.  The final staircase has 10 fewer steps than the second one.  Each step is 0.5 feet.  How many feet did he climb?
The second flight had 20*2=<<20*2=40>>40 steps
The third had 40-10=<<40-10=30>>30 steps
So in total, he climbed 20+40+30=<<20+40+30=90>>90 steps
So he climbed 90*.5=<<90*.5=45>>45 feet
#### 45