If order doesn't matter, then there are C(12, 3) choices, which is \((12 \cdot 11 \cdot 10) \div (3 \cdot 2 \cdot 1)=1320 \div 6=220.\)
However, if order does matter, we count the amount of choices for the first option:12. The second option has 11 choices, and the third, 10. we multiply them to get 1320 choices.
so we know that these four quarters of circles make up a full circle with a radius of 4 cm. So we need to find 36-(3.14*16-overlaping region). So the actual problem is finding the overlapping region. We know that the straight line under the overlapping part is 2, as 4+4-6=2. Can you carry on from there?
For your second post:
1. n must be a negative number, as all positive values of n would be greater than n-3. n must also be greater than -6, as -6-3=-9, and the absolute value of that would be nine, so 0>n>-6, so the possible integer values of n are 0, -1, -2, -3, -4, and -5. Add that up, and you get -15.