mean of a set of numbers is
(sum of the values of the numbers in the set)/(how many numbers in the set)=mean
example
1,2,4
mean is
(1+2+4)/3=7/3
ok
mean of 7 numbers is 21
(7numbers)/7=21
"5 of them are 18,23,21,17"
(mentally do 7-5=2)
2 missing
"one of the remaining is 3/4 of other"
the 2 remaining is x and 3/4x
nmow we have
(18+23+21+17+19+x+3/4x)/7=21
add like terms
(98+1 and 3/4x)/7=21
mulitply everybody by 7
98+7/4x=147
minus 98 both sides
7/4x=49
multiply both sides by 4/7 to clear fraction (7/4 times 4/7=28/28=1, so 1x=x)
1x=196/7
x=28
the other number is 3/4 of this
3/4x=3/4*28=21
the 2 unknowns are 28 arnd 21
