300 - 297 = 3
294 - 291 = 3
288 - 285 = 3
So, that is a sequence of sums of number 3: 3 + 3 + 3 + .... + 3
How may times will the number 3 be added?
Note that 300 is reduced in 6 units each time => 300 / 6 = 50 => 3 will be added 50 times.
=> 50 * 3 = 150
Answer: 150