Why do some chemical symbols for elements consist of 2 letters and others of only one

Since there are only 26 letters in the alphabet and there are (currently) 111 known elements, therefore not every element can be represented by only 1 letter. Also some elements date back to antiquity and already have symbol names assigned by Alchemists, for instance, rather than being G, gold is Au for the Latin word aurum  for shining dawn.

i think slash and

for step 7 table c on the lab "newtons law of action"   the average time it took the car to travel .25 meters   was 1.92 seconds.

the average time it took the car to travel 0.50 meters was 2.61 seconds

hope this : )