Feedwater Flow Question

SteveSteam

Feedwater is delivered to a boiler by means of a line which is approximately 1.5" I.D. In order to deliver twice as much feedwater at the same pipe flow velocity, the APPROXIMATE I.D. of the line should be?

A) 3.00"
2.12"
C) 1.98"
D) 2.98"

I know the answer is 2.12"

But how did they get the answer?

SWEI

1.5² = 2.25
2.25 * 2 = 4.5
√4.5 = 2.12