21/10 more than y is 19/5 CORRECT
17/10 + 21/10 = 38/10 simplified to 19/5
34/5 less than y is -51/10 CORRECT
17/10 - 34/5 = 17/10 - 34/5*2/2 = 17/10 - 68/10 = (17-68)/10 = -51/10
15/2 more than y is 29/5 INCORRECT
17/10 + 15/2 = 17/10 + 15/2 * 5/5 = 17/10 + 75/10 = (17+75)/10 = 92/10
18/5 less than y is -53/10 INCORRECT
17/10 - 18/5 = 17/10 - 18/5 * 2/2 = 17/10 - 36/10 = (17-36)/10 = -19/10