Using slice/mask instead of a for-loop to find items in an array

Question

p= np.array([[ 1,  2,  3,  4,  5],
   [ 6,  7,  8,  9, 10],
   [11, 12, 13, 14, 15],
   [16, 17, 18, 19, 20]])

I have the above array and need to find t