First, some defintions
reciprocals are like a/b and b/a or 5/1 and 1/5
remember,

john's fraction is reciprocal of sarah's
john=a/b
sarah=b/a
david multiplies his by sarah and gets 12/35
(b/a)(david)=12/35
multplies his number by john and gets 12/35
(a/b)(david)=15/7
hmmm
(b/a)(david)=12/35 and
(a/b)(david)=15/7
if we divide them then we do





square root both sides

therefor
b=2 and a=5
john's number is a/b=5/2
sarah's number is 2/5
david, hmm
sarah times david=12/35
2/5 times david=12/35
times both sides by 5/2
david=60/70
david=6/7
check other
john times david=15/7
5/2 times 6/7=15/7
30/14=15/7
15/7=15/7
true
sarah's favorite number is 2/5
john's favorite number is 5/2
david's favorite number is 6/7
2 7/8= 2/3L
Divide both sides by 2/3 and get 4.31 or 4 5/16
Hope this helps!!
It is 20 because the 5 is in the ones place so the 2 has to be in the 10 place which makes it 20
Because vertical angles always equal each other. linear pairs always equals 180 degrees always. hope this helps :)
The answer would be 5.491 million, but can be rounded to 5.5 million.