Radius of droplet=2×10⁻⁵m=0.002cm.

The volume of a droplet is (4/3)(0.002)³π cc. So (4N/3)(0.002)³π=30.

From this N=(45/2)/(0.002)³π)=2.8125×10⁹/π.

The surface area of each droplet is 4π(0.002)²=1.6π×10⁻⁵ sq cm.

Total surface area is 2.8125×10⁹/π × 1.6π×10⁻⁵ sq cm = 4.5×10⁴ sq cm = 45000 sq cm.

(If r=radius of droplet, (4/3)πr³ is its volume. If V is the volume of the gasoline, N=V/((4/3)πr³)=3V/(4πr³). Surface area of droplet=4πr². Total surface area=4πr² × 3V/(4πr³)=3V/r=90/0.002=45000 sq cm. Using algebra first greatly simplifies the calculation.)

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