An aircraft flies through air, and the air is going somewhere itself. Draw the vectors and everything follows: your true airspeed through the air, the wind across the ground, and the resultant — what you actually make over the earth. That drawing is the wind triangle, and an E6B is a circular slide rule built to solve it. It asks two questions and answers them separately.

How fast? $V_g = \sqrt{V^{2} - W^{2}\sin^{2}\theta} - W\cos\theta$. $V$ is the true airspeed in knots (TAS, not the indicated figure on the dial), $W$ the wind speed in knots, $\theta$ the angle between the wind and the intended track — 0° a dead headwind, 180° a dead tailwind — and $V_g$ the ground speed. Read the two terms: $-W\cos\theta$ is the head or tail component, straight off the front or the back, and the root term is what the crosswind quietly costs on top, because you have to crab into it.

Which way? $\mathrm{WCA} = \arcsin\!\left(\dfrac{W\sin\theta}{V}\right)$ — the wind correction angle, or crab angle: how far off the desired track to point the nose so the wind carries you along it instead of off it. Only $W\sin\theta$, the crosswind component, appears. The along-track part of the wind changes your speed and never your heading, and that separation is the whole elegance of the thing.

Then $t = \dfrac{d}{V_g}$ turns it into an arrival time — and the denominator must be the GROUND speed. True airspeed in a headwind gives a time that is confidently wrong, always in the optimistic direction, on the leg where it matters most. A fuel reserve computed against that number inherits every bit of the error.

One more thing about the order of operations, because it catches people: the crab angle is applied to the true track to get a true heading. Variation and deviation come afterwards, to turn that true heading into something the compass can steer. Wind first, then the norths.