The opportunity cost of increasing the production of good A from 10 units to 20 units is the number of units of good B that must be given up to produce the additional 10 units of good A. If the economy is currently producing 10 units of good A and 90 units of good B, the opportunity cost of producing one additional unit of good A is 90/10 = 9 units of good B.
So, the opportunity cost of increasing the production of good A from 10 units to 20 units is 9 units of good B per unit of good A * 10 units of good A = 90 units of good B.