Races – Complete GMAT Guide
Race questions are simply Distance = Speed × Time questions in disguise.
Most race questions become easy if you understand:
• Head starts
• Dead heats
• Beating by distance
• Beating by time
• Speed ratios
- Always Draw a Track
Visualize the race.
Example:
Start -------------------------------- Finish
Whenever a head start is given, place racers accordingly.
Why?
Race questions are easier when you think in terms of positions rather than equations.
- Two Types of Head Start
Type 1: Distance Head Start
A gives B a 10 m head start in a 100 m race.
Initial positions:
Start A----B---------------------------Finish
A runs 100 m.
B runs only 90 m because B starts 10 m ahead.
Key idea:
Both start at the same time.
They do not start from the same position.
Type 2: Time Head Start
A gives B a 1 minute head start.
Timeline:
12:00 B starts
12:01 A starts
Both start from the same position.
Key idea:
They start at different times.
They do not start from different positions.
Quick distinction:
Distance head start:
• Same time
• Different positions
Time head start:
• Different times
• Same position
- Dead Heat
A dead heat means both racers finish at exactly the same time.
Finish
A |
B |
Key translation:
Time of A = Time of B
Most race questions eventually create a dead heat condition.
Whenever you see:
"finish together"
"tie"
"dead heat"
Immediately think:
Time of A = Time of B
- Two Ways to Beat Someone
A Beats B By Distance
Suppose race length = 1000 m.
A beats B by 20 m.
When A reaches the finish line:
Finish
A |
B is still 20 m away from the finish line.
Meaning:
When A has run 1000 m,
B has run only 980 m.
Important:
Always freeze the race when the winner finishes.
Do not think about when B eventually finishes.
A Beats B By Time
Suppose A beats B by 2 minutes.
If A finishes at 12:10,
B finishes at 12:12.
Meaning:
At 12:10, B still requires 2 more minutes at B's own speed to finish.
Key difference:
Beats by distance → compare positions.
Beats by time → compare finishing times.
- Golden Rule of Race Questions
Speed ratio tells you distance ratio for equal times.
If:
A:B = 20:17
then in the same amount of time:
A covers 20 units.
B covers 17 units.
Examples:
A = 20, B = 17
A = 40, B = 34
A = 200, B = 170
This single idea solves most race questions.
Important:
Speed ratio gives distance ratio only when time is equal.
Do not treat it as a time ratio.
- Dead Heat Using Speed Ratios
Example:
A's speed is 20/17 times B's speed.
Therefore:
A:B = 20:17
Interpretation:
In equal time:
A covers 20 units.
B covers 17 units.
Suppose race length is 20 units.
When A reaches the finish line:
A = 20
B = 17
B is behind by:
20 − 17 = 3 units
To make them finish together, B should start 3 units ahead.
Start
A---B-----------------
Then:
A still runs 20 units.
B runs only 17 units.
Since 20 and 17 take the same time at speeds 20:17, they finish together.
Therefore head start:
3/20 of race length
Shortcut:
If speed ratio:
A:B = a:b
and race ends in a dead heat,
Head Start = (a − b)/a
Example:
A:B = 20:17
Head Start = (20 − 17)/20 = 3/20
- Beating By Distance
This is the most common race setup.
Example:
A:B = 20:17
B must beat A by 20% of the race length.
Step 1
Choose race length = 25.
Why?
20% of 25 = 5
Easy calculations.
Step 2
If B beats A by 20%,
then when B finishes,
A is still 5 units away from the finish line.
Therefore A has covered:
25 − 5 = 20
units.
Step 3
Speed ratio is 20:17.
So whenever A covers 20 units,
B covers 17 units.
Step 4
But B must actually be at the finish line, i.e. at 25 units.
Therefore B needs an extra:
25 − 17 = 8
units.
That extra distance must come from a head start.
Head Start = 8/25
Head Start Percentage = 32%
Answer = 32%
Important Visualization
Whenever a runner beats another by x meters:
Freeze the race at the exact moment the winner finishes.
Then ask:
Where is the loser?
This visualization prevents most mistakes.
- Standard GMAT Race Framework
Step 1
Convert speeds into a ratio.
Example:
20/17 becomes 20:17
Step 2
Imagine equal time.
Ask:
How much distance does each cover in the same time?
Step 3
Use a convenient race length.
Often use:
20
25
100
L
Step 4
Draw starting positions.
Examples:
Start A---B---------
or
Start A,B-----------
Step 5
Draw ending positions.
Example:
A finished
B 3 m behind
or
B finished
A 20% behind
Step 6
Translate the picture into an equation.
Most race questions become straightforward after the picture is clear.
Common GMAT Traps
Trap 1: Confusing Time Head Start and Distance Head Start
Always ask:
Did B start earlier?
or
Did B start ahead?
Trap 2: Using Speed Ratio as Time Ratio
Wrong.
Speed ratio gives distance ratio only when time is equal.
Trap 3: Not Freezing the Race
For:
"A beats B by 20 m"
Freeze the race when A finishes.
Do not think about what happens afterward.
Trap 4: Using Actual Race Length Immediately
Often easier to choose:
20
25
100
instead of using the real length.
Most Important Formulas
Dead Heat
If:
A:B = a:b
Head Start = (a − b)/a
A Beats B By Distance
If:
A:B = a:b
then when A finishes,
B has completed:
b/a
of the race.
Distance by which A beats B:
1 − b/a
= (a − b)/a
of the race.
Quick Memory
If:
A:B = 20:17
then whenever A finishes,
B is short by:
20 − 17 = 3 parts.
This is the single most useful race observation for GMAT questions because many race problems reduce to finding those "missing parts."