Although there are several ways to solve this ,
I will not get into those 'x' business.
We know 4/5 of the children are boys therefore the remaining 1/5th must be girls.
Now it is being said the no. Of girls is 200.
Hence , if 1/5th corresponds to 200.
Then the whole part or the total number of students will be equal to 200*5=1000.
And the number of boys would be 1000–200= 800.
*This numerical problem resembles the current situation of engineering colleges in India* XD
hence, ' beti bachao beti padhao.'
Peace out.