Answer:
the number of single room is 27 and the number of double room is 53 
Explanation:
Let x is the number of single room
Let y is the number of double room 
We know that: 
x + y = 80 <=> x = 80 -y (1) 
80x +90y= 6930 (2) 
Substitute (1) in (2) we have: 
80 (80 - y) + 90y = 6930 
<=> 6400 - 80y +90y = 6930 
<=> y = 53 
> x = 80 - 53 = 27 
the number of single room is 27 and the number of double room is 53