Question 972438
average = total/number.

Average is 0.84, number is 20, so total is 16.8

Let x = number of perfect surveys needed.

{16.8 +x}/(20+x)  = 0.91

multiply by (20+x) both sides

16.8 +x = 18.2 +0.91 x
-1.4 =-0.09 x

need between 15 and 16 surveys, so round up to 16 surveys.

Check
36 will be the number of surveys.
The total will be 16.8+12=32.8

32.8/36=0.9111 (repeat)

Try 35 surveys (15 more)
31.8/35 =0.9086.

You need 16 more surveys.