Calculate the sum of all integers from 1 to 300 inclusively. It is= 45150. (1) (use the known formula for the sum of arithmetic progression. See the lesson Arithmetic progressions in this site). Next, calculate the sum of the integers from 0 to 300 that are multiples of 3. It is 3*(1 + 2 + 3 + . . . + 100) = = 3*5050 = 15150. (2) Again, use the formula for the sum of AP. As a final step, distract (2) from (1): 45150 - 15150 = 30000. Answer. 30000.