Change in the simple interest rate changes the interest by a known amount: find the sum

This type of question, asked 3 times: CDS 1 2018, CDS 1 2020, CDS 1 2021. All 3 questions below, oldest first.

Try each one first, then open its answer.

Question 1
CDS 1 2018
35 $The annual income of a person decreases by Rs. 64 if the annual rate of interest decreases from 4% to 3.75%. What is his original annual income?$
Tap an option to check your answer.
Question 2
CDS 1 2020
32 $A sum was put at simple interest at certain rate for 2 years. Had it been put at 1% higher rate of interest, it would have fetched Rs. 24 more. What is the sum ?$
Tap an option to check your answer.
Question 3
CDS 1 2021
32 $A sum of money was invested at simple interest at a certain rate for 5 years. Had it been invested at a 5% higher rate, it would have fetched Rs. 500 more. What was the principal amount?$
Tap an option to check your answer.

Question 1