Data sufficiency gives a question and two statements, and asks whether the statements are enough to answer it. The five answer choices never change, and neither does the discipline: you are deciding sufficiency, not computing a value.
Work the statements separately first, and mean it. Assess statement 1 alone. Then deliberately forget it and assess statement 2 alone. The commonest error in the whole item type is letting information from the first statement leak into your reading of the second — and it produces the answer "both together", which is why "both" is the most over-chosen option.
Sufficiency means ONE answer, not a nice answer. A statement that narrows the value to 4 or −4 is not sufficient when the question asks for the value. A statement that proves x is positive IS sufficient when the question only asks whether x is positive.
Stop the moment it is settled. Once you know a statement determines the answer, move on. Doing the arithmetic afterwards is time spent buying nothing, and across twenty questions it is the difference between finishing and not.
Yes/no questions are different from value questions. For a yes/no, sufficiency means always yes or always no. A statement that sometimes gives yes and sometimes no is insufficient — and that is exactly what testing 0, 1, a fraction and a negative will reveal.
A habit worth building. Before reading either statement, write down what you would need in order to answer — "I need the rate, or both the distance and the time". Then each statement is checked against a list rather than judged by feel.