Maths question (1 Viewer)

[Wispybadboy]

Sup, here's a maths question I pose to you guys:

"The average weight of boys in a group was calculated. David who weighed 39kg joined the group and the figure changed to 51kg. Then Ben, weighing 61kg joined the group as well as David the average changed to 52kg. What was the average weight of the group before David and Ben came along?"

Thanks.

 

[frenzal_dude]

Sup, here's a maths question I pose to you guys:

"The average weight of boys in a group was calculated. David who weighed 39kg joined the group and the figure changed to 51kg. Then Ben, weighing 61kg joined the group as well as David the average changed to 52kg. What was the average weight of the group before David and Ben came along?"

Thanks.

x=initial average weight, b=initial no. of people, w=initial total weight

x=w/b

51 = (w+39)/(b+1) ---> Eq1
52 = (w+39+61)/(b+2) = (w+100)/(b+2) --->Eq2

So now you have 2 simultaneous equations, you can work out b, then sub b back into the formula to get w, then once you have b and w you can work out x.

51 = (w+39)/(b+1)
b+1 = (w+39)/51
b = (w+39)/51 - 1

Now, sub b into Eq2:

52 = (w+100)/[[(w+39)/51 - 1] + 2] = (w+100)/[[(w+39)/51 - 51/51] + 2]
= (w+100)/[(w-12)/51 + 102/51] = (w+100)/[(w+90)/51] = [(w+100)51]/(w+90) = (51w+5100)/(w+90) therefore 51w+5100 = (w+90)52 = 52w+4680 therefore 5100 = w+468

w=420

Now, sub w into Eq1:

51 = (420+39)/(b+1) therefore b+1 = 459/51

b=8

Now we have w and b, sub these back into x = w/b

x = 420/8 = 52.5

x = 52.5kg

 

[Wispybadboy]

Thanks!