A shopkeeper gives a discount of 10% on the marked price and then charges sales tax on the amount obtained. He charges a % sales tax up to Rs. 5000 and b% tax for orders with marked price more than Rs. 5000. If a customer pays Rs. 180 as sales tax when he purchases goods worth Rs. 4000 marked price, and Rs. 189 when he purchases goods worth Rs. 7000 as marked price, find the value of (a + b).

• A. 6
• B. 7
• C. 8
• D. 9
• E. None of these