Let number of nickels and quarters be x and y respectively.
given,
x + y = 34 --------------------(1)
and x/20 + y/4 = 3.9 ------------(2)
from (1) we get:
x = 34 - y --------------------------(3)
putting the value of x from (3) into (2) we get:
(34-y)/20 + y/4 = 3.9
on multiplying both side by 20 we get:
20(34-y)/20 + 20y/4 = 20*3.9
or 34 - y + 5y = 78
or 34 + 4y = 78
or 4y = 78 -34
or 4y = 44
or y =11
So there are 11 quarters in the jar.