１條數學題

1. average typing speed = total words / total time

the time used for typing 1000 words at a speed of 30 words/min:
speed = words/ time
time = words /speed
= 1000/30
= 100/3 mins

the time used used for typing 750 words at a speed of 40 words/min:
time = words /speed
= 750/40
= 75/4 mins

the average typing speed
= total words / total time
= (1000 + 750 ) / (100/3 + 75/4)
= 33.6 words / min

therefore, the average typing speed = 33.6 words/ min

Tom typed 1000 words at a speed of 30 words/min.
This took him 1000/30 mins = 33.33 mins.

Tom then typed 750 words at a speed of 40 words/min.
This took him 750/40 mins = 18.75 mins.

He typed totally 1750 words in 52.08 mins.
Average typing speed = 1750 words / 52.08 mins = 33.6 words/min