Answer:
Explanation:
Usually heart rate is talked about in beats per minute, but they measured their heart rates over 10 seconds and 15 seconds for this reason:
Starting with the 15 sec measurement, there are 4 "15 seconds" in 1 minute, right? 15 sec is a quarter of a minute, just like 15 minutes is a quarter of an hour, which is why we talk about time like "quarter after" or "quarter till."
So if Kathy counted 21 beats in 15 seconds, then we can extrapolate that to 84 beats in 1 minute, or 84 bpm. You can do that because you know you can multiply 15 sec by 4 to get 60 sec, which is 1 minute. Then you have to multiply the 21 beats by 4 also, to get 84.
Carey's measurement is similar, but how many "10 seconds" are there in a minute? 6, right? So multiply her 15 beats by 6 to get 90 bpm.
And then you can see that Carey's heart rate, the beats per minute, is higher than Kathy's.
That's the fast/easy way, and that's why the girls counted over 10 or 15 seconds. But you can do it with "math" too:
15 beats in 10 seconds is this:
15 beats/10sec, which equals 1.5 beats/sec
Then multiply by 60 seconds per minute to get bpm:
1.5 beats/sec x 60 sec/min = 90 bpm
Same answer as before. Do you see how "seconds" cancelled out, leaving "beats" over "minutes."
You can do Kathy's the same way.