Răspuns :
4200 + 4200·15 + (4200+4200·15):2 + x = 140000
4200 + 63000 + (4200+63000):2 + x = 140000
67200 + 67200:2 + x = 140000
67200 + 33600 + x = 140000
x = 140000 - 67200 - 33600
x = 39200
4200 + 63000 + (4200+63000):2 + x = 140000
67200 + 67200:2 + x = 140000
67200 + 33600 + x = 140000
x = 140000 - 67200 - 33600
x = 39200