A 500-gallon water tank draining at 25 gallons per hour hits empty when the line 500 − 25x crosses zero — at x = 20 hours. The x-intercept is where a graph crosses the horizontal axis: the moment the quantity runs out, the break-even point on a timeline, the root of the equation. For a line y = mx + b, setting y = 0 gives x = −b/m — the intercept is the starting value divided by the rate, with a sign flip. The tank's −500/−25 = 20 hours is the number the maintenance crew schedules around.

Since the intercept is (−b) × (1/m), this calculator multiplies those two pieces. Type −b (the negated starting value) in the Variable A box — for y = −25x + 500, b is 500, so enter −500 — and 1/m in the Variable B box (−1/25 = −0.04). The Result box shows (−500) × (−0.04) = 20: the tank runs dry at hour 20. Economists finding break-even timelines and chemists reading titration endpoints use the same two-box move.

With the pre-filled defaults — Variable A 10, Variable B 5 — the X Intercept Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.