Two friends A and B went to fruit market to buy some fruits. At first shop B bought some mangoes at ₹ a per mango and A bought 9 mangoes more than
B. At the next shop A bought some apples at a unit price of ₹ b per apple and B bought 18 apples more than
A. Later, it was found that the ratio of number of fruits bought by A and B is 4: 7, respectively, whereas the amounts of money spent by them to buy fruits are the same. Find the ratio a:
b.