Question

what fraction must be subracted from the sum of one by four and one by six to have an average of one by twelve of all the three

Answer 1

Sum = 1/4 + 1/6 = 5/12

Average of the three numbers :  1/4,  1/6,  x   be   1/12
so  1/4 + 1/6 + x = 1/12 * 3 = 1/4
=>     x = -1/6

After subtraction of some quantity from the sum 5/12 we get  -1/6.

To obtain -1/6 from the sum 5/12 we have to subtract 5/12 - (-1/6)
   = 7/12.