Two pipes A and B can fill a cistern in 37 x 1 /2 minutes and 45 minutes respectively.

So B is turned off after x minutes 

So for x minutes A and B work together and for 30-x minutes A works so the cistern is filled. 

A can fill the cistern in 37 12 min=752 min

So A can fill in one minute 1752=275 of the cistern 

B can fill the cistern in 45 min

So B can fill in one min 145 of cistern 

So in one minute A and B together can fill =275+145=115(25+13)=115(6+515)=11225

By the condition, 

11x225+275(30-x)=1

11x225-2x75+45=1

11x-6x225=1-45

5x225=15

x=152255=9

Hence the value of x is 9.