past x laps

[shibal]

for the past x laps around the track, steven's avg time per lap was 51 seconds. If a lap of 39 seconds would reduce his avg time per lap to 49 sec, what is the value of x?

OA: 5

[truplayer256]

If Steven's average time per lap for the past x laps was 51 seconds, then his total time for the x laps would be 51*x seconds. From the information given in the problem, we know that if Steven runs 39 seconds on a lap, his average will go from 51 to 49. We can set up an equation:

(51x+39)/x+1=49
51x+39=49x+49
2x=10
x=5