Hi,
To find the value of the number we have to first find the value of the single percantage of this number. We can do so by dividing 35 with 20.

The value of single percantage would be 1.75.
Mathematically: 1% = 1.75
Now to get the value of 50% we simply multiply the value of 1% with 50.

So 50% of unknown number is 87.5 that would be the ANSWER (D).
Hope this helps.
If you posses and other questions related to the answer leave a comment and I will reply.
Cheers, r3t40