Childrens Mystery series. Multiple "solve it" short stories per book. 4 siblings (girls & boys) solve mysteries in pairs. Mystery at carnival; mystery at old widow's mansion. Cover: dark green, magnifying glass with book #. Read before late 2000's; 1990's?

[Eli]

there was also part of a you solve it mystery at the end of each book, before late 2000's, probably 90's. In one book they solve a mystery at a carnival, in another a mystery at an old womens mansion, I think her husband had died. Thanks so much! I've looked for hours!

[Crystal]

Is this book a picture book or a novel?

If it's a novel it sounds like it could be something by Enid Blyton - she wrote many stories with boys and girls solving mysteries together.

The Find Outers series has girls and boys (who split up into teams to solve mysteries). There are too many different cover designs over the years but some of them have magnifying glasses on it:
The Mystery of the Strange Bundle


The Adventure series also has girls & boys teaming up:
The Circus of Adventure

Plus there is also
Secret Seven
The Secret Seven

Famous Five
Five Go to Mystery Moor

^For these books you're going to need to check out the entire series not just the book I linked (the booked I linked is probably not the right one but it might be one from the correct series. Enid Blyton wrote too many books and I can't remember specifically which one featured the carnival & mansion - especially since too many of her books feature these two themes

[Eli]

It was a chapter book, there were a few pictures per story but in a "solve it" fashion, and I thought I mentioned it but there were multiple short stories, like novellas, in each book, but one of them always corresponded with the title

[Kris]

Eli, I added some details to the header/topic title. You can update it by clicking the small "edit" link after the header. This only works on the full Desktop website - not the Mobile website or app. (On the Mobile website, click the "Desktop version" link at the bottom of the page.)

[Crystal]

oooh! If it has multiple stories inside it's probably not one the Enid Blyton mystery solving books

Will have to think and see if I can remember anything else that fits

[Valerie]

Long shot, perhaps the T*A*C*K series? First one is T*A*C*K Secret Service