100% premium from $7.5=$15:
40M S/O,premium paid=$7.5*40=$300M;400M S/O,premium paid=400M*7.5=$3B
150% premium from $7.5 = $18.75:
40MS/O,premium paid=$11.25*40M=$450M;400MS/O,premium paid=$11.25*400M=$4.5B
$4.5B-$3B >> $450M - $400M, i.e., law of large number