CalTheGreat

"To find the harmonic mean of a set of n numbers, add the reciprocals of the numbers in the set, divide the sum by n, then take the reciprocal of the result." -www.mathworlds.com

So, to find the harmonic mean of 45 you'd do: 1/((1/45+1/63)/2)

its fine for me....

girls: 520 times 0.4 equals 208 girls.

1/2 of othat is 104. 

104 girls travel by bus. 

60 percent are boys.

520 times 0.6 is 312 boys.

312 boys travel by bus.

add that and get 416.

your name shows in the side of the leaderboard, and you get a ton of satisfaction c:

i think that if your doing basic math, sure, do it in your head if its ezer for you.

but if your doing advanced math, do. it. on. paper. because when you do the complicated stuff and your brain explodes with stress, you make simple mistakes when doing stuff like this in your head.