The old volume was:

2 x 6 x 10 = 120 cubic inches

The new volume is:

3 x 5 x H = 120 cubic inches

15H = 120

H = 120/15

H = 8 inches

2 x 6 x 10 = 120 cubic inches

The new volume is:

3 x 5 x H = 120 cubic inches

15H = 120

H = 120/15

H = 8 inches