A tank is filled in 45 minutes by two pipes, A and B. Pipe B fills the tank twice as fast as A. How much time (in minutes) will pipe A alone take to fill the tank?

Correct — B. Work in rates, not in times.

Let A fill 1 unit of tank per minute. B is twice as fast, so B fills 2 units.
Together that is 3 units a minute, and the pair finishes in 45 minutes.
A alone therefore needs 3 × 45 = 135 minutes.

Check: 1⁄135 + 2⁄135 = 3⁄135 = 1⁄45 → option (b).

Why the others are wrong

• (a)
  115 — 115 minutes for A puts B at 57.5 and the pair at 3⁄115 of the tank a minute, a joint fill of 38⅓ minutes instead of the 45 given.
• (c)
  125 — 125 minutes gives a joint rate of 3⁄125, filling the tank in 41⅔ minutes. Every wrong option here fails this same one-line check.
• (d)
  140 — 140 minutes gives 3⁄140 a minute, so the pair would take 46⅔ minutes — past the 45 minutes the question fixes.

"Twice as fast" is a statement about rate, and rates add. If A fills 1 unit a minute then B fills 2, and the pair fills 3.

Times do not add. A alone takes three times as long as the pair precisely because the pair works three times as fast.

A supplies one third of the joint rate, so A alone takes 3 × 45 = 135 minutes, and B, supplying two thirds, takes 1.5 × 45 = 67.5 minutes.

Key facts

• Rates add and times do not: 1⁄A + 1⁄B = 1⁄45 here.
• B twice as fast as A means A supplies exactly one third of the combined rate.
• A alone takes 135 minutes and B alone 67.5 minutes.

Common traps

• Reading "twice as fast" as "takes twice the time", which puts A at 67.5 minutes.
• Doubling or halving the 45 minutes directly instead of splitting the joint rate.

No directly related past PYQ was found.