Concepts
Featured · MCAT physiology: Nernst and Goldman
A patient's potassium doubles. A four-minute film that builds the resting potential from nothing, ion by ion, and shows why the membrane moves nine millivolts, not eighteen, and why that is enough to stop a heart.
Pause, scrub, read alongEvery frame is computed
Pick up where you left off
Five SAT problems, three from the MCAT, two LSAT and two from the CFA and Series 7, each built around a real test question: the traps, the one picture that makes the idea obvious, the general rule. Every film has an interactive version where the numbers change and the set redraws itself.
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MCAT physiology: Nernst and Goldman
A neuron rests at -70 mV with [K+] = 4 mM outside and 140 mM inside, [Na+] = 145 mM outside and 15 mM inside. In a patient with hyperkalemia the extracellular potassium doubles to 8 mM. What happens to the resting membrane potential?
3:28
LSAT logic games: ordering
Six speakers, F, G, H, J, K and L, speak one at a time in slots 1 to 6. K speaks third or fourth. J speaks immediately after F. G speaks before H. L does not speak first. If L speaks second, which one of the following must be true? (A) F speaks fourth (B) H speaks sixth (C) G speaks first (D) K speaks third (E) J speaks fifth
4:13
CFA Level I: bond duration and convexity
A 3-year annual-pay bond with a 5% coupon is priced at par to yield 5%. Yields rise to 6%. Using duration, the approximate percentage change in price is closest to: (A) -1.00% (B) -2.72% (C) -2.86% (D) -3.00%
3:21
MCAT fluids: Poiseuille's law
A plaque narrows a coronary artery to half its original radius. If the pressure difference across the segment and the viscosity of the blood are unchanged, blood flow through the segment falls to what fraction of its original value?
3:45
LSAT logical reasoning: conditional logic
Every student who completed the seminar received a certificate. Maria received a certificate. Therefore, Maria completed the seminar. The reasoning is flawed because it: (A) treats a condition sufficient for a certificate as if it were necessary (B) treats a necessary condition as sufficient (C) draws a conclusion about one student from a claim about all (D) presumes what it sets out to prove (E) mistakes a correlation for a cause
3:53
Series 7 / CFA: option payoffs
An investor buys a share of stock at $50 and buys a $45 put for $2. At expiration, the maximum loss per share is: (A) $2 (B) $5 (C) $7 (D) unlimited
4:31
MCAT biochemistry: enzyme inhibition
An enzyme has Km = 2 mM and Vmax = 100 µmol/min. A competitive inhibitor raises the apparent Km to 6 mM. At [S] = 2 mM, the rate in the presence of the inhibitor is closest to: (A) 50 (B) 33 (C) 25 (D) 17 µmol/min.
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Rates: work problems
Alice can paint a room in 6 hours. Bob can paint the same room in 3 hours. Working together, how long will it take them to paint the room?
4:26
Systems of linear equations: distance, rate and time
A freight train leaves a station heading east at 40 miles per hour. Two hours later an express train leaves the same station on a parallel track heading east at 60 miles per hour. How many hours after the express leaves does it catch the freight train?
4:05
Average speed: total distance over total time
A cyclist rides from her house to the beach at 20 miles per hour and rides back along the same road at 30 miles per hour. What is her average speed for the whole round trip?
4:06
Circles: completing the square
In the xy-plane, the graph of x² + y² + 6x − 4y = 12 is a circle. What is the radius of the circle?
3:50
Similar triangles: proportions
At a certain time of day, a flagpole casts a shadow 36 feet long. At the same time, a person 5 feet tall standing nearby casts a shadow 4 feet long. How tall is the flagpole?
Five rounds each: sliders, multiple choice and a trap round, with the set showing the truth after every answer. A sandbox tab lets you change the numbers freely. Scores are remembered in this browser.
The Resting MembraneGoldman
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Six Speakers, Six SlotsLogic games
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The Balance PointDuration
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The Narrowed ArteryPoiseuille
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The One-Way DoorConditionals
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The Floor Under the StockOptions
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The Crowded Active SiteEnzyme kinetics
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Two Painters, One RoomRates
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Two TrainsSystems
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The Round TripAverage speed
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The Unfinished SquareCircles
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The ShadowSimilar triangles
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Every frame is a function of the clock, so a film can be paused, scrubbed, re-timed to a new narrator, or re-rendered with different numbers. The same engine drives the film and the practice set.
Each film follows the shape of a good tutor's explanation: state the question, show why the tempting answers fail on the set itself, then let one camera move carry the argument.
A tutor beside the set: ask "what if Bob were slower?" and watch the room answer. The practice engine already takes the numbers; the conversation is the layer on top.