Question

suppose the economy is currently producing 15 tanks and 55 machine guns.what is the opportunity cost of producing 60 machine guns

Answer 1

Now you are producing extra five machine guns that means you are using resources that was used to produce 15 tanks. Since some resources are used to produce extra five guns, you will not have resources to produce 15 tanks, you will be producing less than 15 tanks.
For example, if resources needed to produce 1 tank can produce five machine guns then the opportunity cost of producing 60 machine guns is producing 14 tanks i.e producing one less tank.

Answer 2

Now you are producing extra five machine guns that means you are using resources that was used to produce 15 tanks. Since some resources are used to produce extra five guns, you will not have resources to produce 15 tanks, you will be producing less than 15 tanks.
For example, if resources needed to produce 1 tank can produce five machine guns then the opportunity cost of producing 60 machine guns is producing 14 tanks i.e producing one less tank.