Question

ricardo ran the 400-meter race 3 times ,his fastest time was 54.3 seconds, his slowest time was 56.1 seconds, if his average time as 55.0 second what was his time the third race.

Answer 1

Take the 2 times you are given and add them to x and divide the entire left side of the equation by 3:
54.3+56.1+x/3=55
54.3+56.1+x=165
x=54.6
The 3rd lap took him 54.6 seconds to run