Cost of digging first metre = Rs. 150
Cost of digging first 2 metres = 150 + 50 = Rs. 200
Cost of digging first 3 metres = 200 + 50 = Rs. 250
Cost of digging first 4 metres = 250 + 50 = Rs. 300
Clearly; 150, 200, 250, 300 … forms an A.P. because every term is 50 more than the preceding term.