I need help with a math problem

I need help with a math problem

Q1) Mantua opened a recurring deposit of Rs 400 per month. Bank gives 6% p.a as the rate of interest. If the maturity value of account is Rs 6240, find the tie , In months, of this recurring deposit account.   Q2) A ready name shirt is marked at Rs 750. During sales dealer allows a discount of 20% on marked price. i) when sold at net pricw the dealer makes a profit of 20% on cost. What was the cost price? If allows a further discount of 5% on net sale price, what will be his profit % ? iii) find sales tax on net sale price at 10% .