1. It appears that there are 45 girls and every one of them must be studying at least one subject. If every one of them studied exactly one subject, the sum of those studying history and psychology would be 45. But it's 65, impling that 20 (65-45) are studying both.
C = Number of chestnuts,
2C = Number of black horses (twice as many),
C+2C+1 = Number of brown horses (one more than the sum of the other two)
Since there are 25 altogether, 25 = C+2C+C+2C+1. Solve for C and double it to derive the number of black horses.
3. Everything would appear to be shifted counterclockwise by the same amount. So if North is 12 o'clock, west is 9 o'clock, south is 6 o'clock...you can probably figure out the rest.
4. Algebra again.
A = apples
A = oranges (same number as apples)
2A = mandarins (twice as many as oranges)
A+A+2A = 15 (Are you sure that's not 16?)
The rest of these seem to be similar themed problems. Try the above approach of defining one unknown item as a letter of the alphabet and synchronizing the other items to it, as shown above. As you can see from the above, it should work.
I hope this helps.