Find how long $200 should be left to accumulate at a 7% effective rate in order that it will amount to twice the accumulated value of $200 deposited at the same time at a 4% effective rate.

A manufacturer sells a product to a retailer who has the option of either paying 20% below retail price immediately or 15% below the retail price in ten months. Find the annual effective rate of interest at which the retailer would be indifferent between the two options.