about 432 cubic inches
.21 yards of sand will fill an area 115 inches by 67 inches to a depth of 2.5 inches.
22/7x6x6x6 cubic inches
None, as a hole which is 6 inches wide and 42 inches long has no depth and thus no volume.
6,400 ft3
25 feet is equal to 300 inches. The radius of a hole is half its diameter, which equals 4 inches. The volume of the cylinder is given by the formula v = [Pi]r2h, so the volume of your hole is 3.14 * 16 * 300 = 15080 cubic inches. There are 1728 cubic inches to the cubic foot, so you'll need about 8.75 cubic feet of concrete, which is about a third of a yard. (More exactly, that's a cubic yard, but concrete guys just say "yard.")
18.6240 yd³
To install a mailbox post correctly, dig a hole at least 24 inches deep, place the post in the hole, and fill it with concrete. Make sure the post is level and allow the concrete to set before attaching the mailbox.
42 cubic feet or 1.56 yards.
.21 yards of sand will fill an area 115 inches by 67 inches to a depth of 2.5 inches.
The recommended method for installing a concrete footing for a 6x6 post is to dig a hole that is at least 12 inches in diameter and 24 inches deep. Place a cardboard tube form in the hole and fill it with concrete, making sure it is level. Insert a post anchor into the wet concrete and allow it to cure for at least 24 hours before attaching the post.
22/7x6x6x6 cubic inches
None, as a hole which is 6 inches wide and 42 inches long has no depth and thus no volume.
6,400 ft3
509.7 litres
How deep? 1 inch will take .7 yards
You can use a concrete crack filler to fill concrete cracks.
29.45 cubic feet or 183.45 US gallons.