David and Mark are marking papers each set takes David 61 minutes and Mark 1 hour express the times David and Mark take as a ratio in its simplest form

Look at attachment. has all info needed. need

Use the method of cylindrical shells to find the volume v generated by rotating the region bounded by the curves about the given axis. y = 7ex, y = 7e^-x, x = 1; about the y-axis

What number is 7 units left of -15?