Suppose we have two stacks of 20 coins each, as shown in the figure. Which stack, the right cylinder or oblique cylinder, has the greater volume?

They both have same volume. After all they both have 20 coins and assuming each coin has same volume, then how can 20 coins in 1 stack have different volume than 20 coins in another stack no matter how you stack them

In general two cylinders with same base area and vertical height have same volume even if one is oblique and the other is a right cylinder.

This is like a rectangle and parallelogram with same base and same perpendicular height having same area