Practice UPSC CSAT previous year questions from 2011–2026 classified underNumber System. Review the original question, four options, correct answer and explanation for every PYQ.
2 questions • 2011–2026
If R and S are different integers both divisible by 5, then which of the following is not necessarily true?
The difference is always divisible by 5, the product by 25, and the sum of squares by 5. But the sum of two multiples of 5 need not be divisible by 10 if their quotient integers have opposite parity.
How many numbers are there between 100 and 300 which either begin with or end with 2?
There are 100 numbers from 200 to 299 beginning with 2. There are 20 numbers ending in 2 between 100 and 300. Ten overlap (202, 212, …, 292), so 100+20−10=110.
Attempt questions one by one with UPSC-style marking and immediate explanations.