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intermediate20 min

Blood Relations

Translate a written family description into a relationship chain and answer correctly.

What you'll learn

  • Translate a sentence like 'X is Y's father' into a directional parent-child relationship
  • Determine whether two people are siblings from a shared-parent structure
  • Determine a grandparent relationship by chaining two parent-child links

Prerequisites

Explanation

Blood relation questions describe a family in a sentence or two and ask how two people are related. The entire skill is translating each sentence into a directional link and then chaining links together correctly.

"X is Y's father" means X is a parent of Y -- the relationship points from X down to Y. A common error is drawing it backward, as if Y were X's parent. Once every sentence is converted into a small directed diagram (an arrow from parent to child), the rest is just following arrows.

Siblings share at least one parent. If A and B both have M as a parent, A and B are siblings (or half-siblings, if only one parent is shared) -- the puzzle usually doesn't distinguish full from half unless it says so explicitly.

Grandparent relationships are two hops: if A is a parent of B, and B is a parent of C, then A is a grandparent of C. This is where most errors creep in on longer chains -- "my mother's only son" is not automatically "me" (unless you are that only son); it depends on how many sons your mother has and who's asking.

A classic trap: "Pointing to a photo, Rita said, 'He is the son of my mother's only son.'" Work it step by step. Rita's mother's only son is Rita's brother (assuming Rita is not that son -- the puzzle context makes clear a male relative is being described). The brother's son is Rita's nephew. Each step is a single, unambiguous hop; the puzzle only gets hard when you try to skip steps instead of writing each one down.

Two-generation family chain

A (parent) -> B (parent, child of A) -> C (child of B). A is B's parent and C's grandparent. B and any of A's other children are C's aunt/uncle or siblings of B, depending on the level.

Example

Checking whether two people share a parent in a small family record.

function isSiblingOf(p1, p2, family) {
  if (p1 === p2) return false;
  const parents1 = family[p1]?.parents || [];
  const parents2 = family[p2]?.parents || [];
  return parents1.some((p) => parents2.includes(p));
}
// isSiblingOf('A', 'B', { A: { parents: ['M'] }, B: { parents: ['M'] } }) -> true

Guided exercise

Guided exercise

Write isSiblingOf(p1, p2, family) where family maps each person's name to { parents: [...] }. Return true if p1 and p2 share at least one parent and are not the same person, false otherwise.

Checks: Recognizes a shared parent as siblings · Rejects two people with different parents · plus 1 hidden check

Code editor. Press Escape then Tab to leave the editor if keyboard focus becomes trapped. Press Control+Shift+M inside the editor to toggle Tab-key focus trapping.

Loading editor…

Stuck? Get a hint.

Independent exercise

Independent exercise

Write isGrandparentOf(p1, p2, family) where family maps each person to { parents: [...] }. Return true if p1 is a parent of one of p2's parents.

Checks: Recognizes a genuine two-hop grandparent chain · Does not confuse a direct parent with a grandparent · plus 1 hidden check

Code editor. Press Escape then Tab to leave the editor if keyboard focus becomes trapped. Press Control+Shift+M inside the editor to toggle Tab-key focus trapping.

Loading editor…

Stuck? Get a hint.

Common mistakes

  • Drawing 'X is Y's father' backward, as if Y were the parent of X.
  • Skipping intermediate steps in a multi-hop chain instead of resolving one relationship at a time.
  • Assuming a shared surname or a vague description implies a specific relationship without a stated link.

Knowledge check

Knowledge check

1. Pointing to a photo, Rita said, 'He is the son of my mother's only son.' How is the boy in the photo related to Rita?
2. If A is B's father and C is A's mother, how is C related to B?
3. What is the most common error when translating 'X is Y's father' into a diagram?

Takeaway

Convert every relationship sentence into a directional parent-child link before combining them -- the direction is where most errors happen.

Summary

Blood relation puzzles are solved by translating each sentence into a directed parent-child link, then chaining links to answer sibling, grandparent, or more distant relationship questions.

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