UPSC Maths Problems Challenge : A Problem A Day! (Day 12)

I am going through a very hard schedule indeed (and have also got a terrible headache today) wherein I am having to go through Trigonometry,Factorials and Complex Numbers at the same time.Moreover,Economics is getting more challenging (as it should in higher levels) day by day.I myself have to go through IAS Prelims,Mains (at least what I am capable of doing through my limited knowledge) and Interview questions along with Statistics practice for entrance exams and let me tell you this is from no angle less tough than any other stream,if not one of the hardest.However,I have got to go ahead with today's post and here it is.

(Original Photograph Source)

Today's Problem(24/06/2015)

Question Credit : http://upsciascsat.eduamigo.com/

Find the missing number.

9 + 4 + 2 = 5 + 3 + 7 = 9 + 1 + 5 = 4 + 8 + 3 =15 

Now,8 + 1 + x = 15.Also,2 + x + 7 = 15.

In both ways,we get x = 6.

Therefore, x = 6.So,the missing number will be 6.