Joe’s average (arithmetic mean) test score across 4 equally weighted tests was 70. He was allowed to drop his lowest score. After doing so, his average test score improved to 75. What is the lowest test score that was dropped?
Choose the most appropriate option
A) 20 B) 25 C) 55 D) 65 E) 80

Rationale: "the arithmetic mean of 4 equally weighted tests was 70 . so what we can assume is that we have 4 test scores , each 70 . he dropped his lowest score and the avg went to 75 . this means that the lowest score was not 70 and other three scores had given the lowest score 5 each to make it up to 70 too . when the lowest score was removed , the other 3 scores got their 5 back . so the lowest score was 3 * 5 = 15 less than 70 . so the lowest score = 70 - 15 = 55 answer ( c )"
Answer: C