Key idea: Word problems and translation
Most Class 10 questions on this topic are word problems. The skill is in TRANSLATION:
1. Identify the two unknowns; assign variables (e.g., let x = age of A, y = age of B).
2. Write two equations from the two given pieces of information.
3. Solve by your preferred method.
**Common types**:
- **Age problems**: 'Father is twice as old as son. Five years ago father was three times. Find ages.'
- **Number problems**: 'A two-digit number plus its reverse is 99...'
- **Distance/time/speed**: 'Boat in still water speeds, downstream/upstream rates...'
- **Cost problems**: 'Cost of pencils and pens.'
- **Fraction problems**: 'A fraction becomes 1/2 when 1 is added to numerator and 2 is subtracted from denominator...'
Always CHECK by substituting solution back into the original word problem (not just the equations).