The second day it rained 10*2=<<10*2=20>>20 inches
On the third day it rained 20*1.5=<<20*1.5=30>>30 inches
So in the first 3 days, it rained 10+20+30=<<10+20+30=60>>60 inches
It drained 4-1=<<4-1=3>>3 days
So it drained 3*3=<<3*3=9>>9 inches
That means on the start of the fourth day, there were 60-9=<<60-9=51>>51 inches in the pond
The area can hold 6*12=<<6*12=72>>72 inches
So it rained at least 72-51=<<72-51=21>>21 inches
#### 21