That's 25, not 50... 7 + 15 + 3 = 25 and 4 + 21 = 25 ...Think about it for a minute :)
He he, yeah funny that you mention it. I wrote a quite long edit update post about this just minutes after my previous post, but Internet Explorer crashed and wiped everyting. I was at work and didn't have time to rewrite it. But yeah, as I realized myself, a daily or weekly challenge is wither a Spartan Ops, War Games or Campaign challenge at the same time, thus making it 50.
So your calculation is quite correct. Strange why 343i would display them as "50" instead of dividing them into two 25's. Like
You have completed
divided as follows
War Games: 7
Spartan Ops: 15
That would make more sense imho :)