Twelve decreased by half a number is 6. Find the number.

12

Step-by-step explanation:

12-n/2=6

-n/2= -6

n/2=6

n=12

41

Step-by-step explanation:

Lets say blue marbles are B and yellow marbles are Y .

We know Y is twice B plus one, so Y = 2*B + 1

We also know that there are 61 marbles in total and there are only blue and yellow marbles, so

Y + B = 61 .

But wait, disnt we say Y = 2*B+1 ? Then we can rewrite that to:

2*B + 1 + B = 61

If we remove 1 from both sides, we get:

2*B + B = 60

Since 2*B = B+B, we know that B+B+B would be 3*B

So now we have 3*B = 60.

We divide both sides by 3 and we get B = 20

And then we go way back to Y being twice B plus one.

If B is 20, twice B would be 40. Plus one, 41.

So Y = 41

P.S. I found this on the internet!