Atelevision commercial claims thta 9 out 10 dentists surveyed recommended bliss toothpaste. if 450 of the dentists surveyed recommended bliss, then how many did not recommend it?

The answer is:  "50 [of the dentists surveyed]" .

Note:  9/10 = 0.9 =0.90 = 90/100 = 90%

Let "x" representation total number of dentists surveyed.

90% of "x"  = 450

(0.9)x = 450 ;

Multiple EACH SIDE of the equation by "10" ; 

9x = 4500 ; 

Dived each side of the equation by "9" ; 

9x / 9 = 4500/ 9 ;

x = 500 ;  

So; 500 dentists total in survey ;  450 out of 500 recommended Bliss ;

The number that did NOT recommend "Bliss" =  500 - 450 = 50 .

The answer is:  "50 [of the dentists surveyed]" .