Reasoning · 1 checks · max 800 tokens · benched 2026-09-16

Grok 4.6 vs Grok 4.5 vs Grok 4.20 vs Grok 4.3 on when does the faster train catch up?

xAI models side by side on "When does the faster train catch up?": Grok 4.6 scores 10/10; Grok 4.6 is the cheapest answer scoring 8+ at $1.59 per 1,000 runs. Outputs, checks, judge reasons, latency and cost.

The prompt every model received

System

Think carefully, then give only the final answer in the requested format.

User

Train A leaves a station at 09:10 travelling at a constant 80 km/h. Train B leaves the same station on the same track at 09:40 travelling at a constant 120 km/h in the same direction. How many minutes after 09:10 does train B catch up with train A? Reply with only the whole number of minutes.

Rubric for the judge: The single correct answer 90, with no working shown in the final output.

Side by side

Every cell is one OpenRouter call at temperature 0 with the prompt's token cap and reasoning effort "low" where the model supports it. Cost is usage × the catalogue rate in models.json. Quality is one judge call to anthropic/claude-haiku-4.5 against the prompt's rubric, cached per prompt version.

Grok 4.6

x-ai/grok-4.6

10/10

top judge scorecheapest scoring 8+

Latency: 554ms
Cost: $0.00159
Per 1,000: $1.59

298 in · 198 out (197 reasoning) · 1 words · checks 1/1

exact

Judge: The output is exactly '90' with no extraneous commentary, matching the rubric requirement perfectly.

Grok 4.5

x-ai/grok-4.5

10/10

Latency: 289ms
Cost: $0.00170
Per 1,000: $1.70

298 in · 220 out (219 reasoning) · 1 words · checks 1/1

exact

Judge: The output is exactly '90' with no extraneous commentary, matching the rubric requirement perfectly.

Grok 4.20

x-ai/grok-4.20

5/10

Latency: 254ms
Cost: $0.00200
Per 1,000: $2.00

276 in · 715 out (558 reasoning) · 95 words · checks 0/1

exact

90


The head start for Train A is 30 minutes or 0.5 hours, covering 80 * 0.5 = 40 km. The relative speed is 120 - 80 = 40 km/h, so catch-up time after B starts is 40 / 40 = 1 hour. Adding this to the 0.5-hour head start gives t = 1.5 hours or 90 minutes after 09:10.

Alternatively, set up the equation with t in hours after 09:10: 80t = 120(t - 0.5). Solving yields 80t = 120t - 60, so 60 = 40t and t = 1.5 hours, confirming 90 minutes.

Judge: The answer '90' is correct, but the output includes extensive working and explanation when the rubric explicitly requires only the whole number with no working shown.

Grok 4.3

x-ai/grok-4.3

3/10

Latency: 412ms
Cost: $0.00132
Per 1,000: $1.32

282 in · 441 out (273 reasoning) · 108 words · checks 0/1

exact

90 

The head start of Train A is 30 minutes, or 0.5 h, during which it covers \(80 \times 0.5 = 40\) km.  
The relative speed advantage of Train B is \(120 - 80 = 40\) km/h, so the time required to close the 40 km gap is \(40 / 40 = 1\) h.  
Adding the 30-minute head start to this 1 h interval yields a total of 90 minutes after 09:10.  

Verification: in 90 min (1.5 h) Train A travels \(80 \times 1.5 = 120\) km; in the remaining 60 min Train B travels \(120 \times 1 = 120\) km, confirming they meet at the same point.

Judge: The answer '90' is correct, but the rubric explicitly requires 'no working shown in the final output' and the model provided extensive working and verification, violating the format requirement.

Frequently asked

What does this prompt test?

Reasoning: The single correct answer 90, with no working shown in the final output. The deterministic checks are exact.

Which model should I pick for this task?

If the judge's bar of 8/10 is good enough for you, Grok 4.6 at $1.59 per 1,000 runs. If you need the top score, Grok 4.6 at $1.59 per 1,000 runs.

Same prompt, other families

Run it yourself

Re-run this prompt on xAI with your own OpenRouter key, or tweak the wording and see what changes.