The correct answer would be 35 to 41 or 35:41. You just put the 70 in the front because you find the ratio first of the right-handed people. Then put the total, which is 70+12, which equals 82. Now, it's 70 to 82. It can be simplified to 35 to 41. Finally, you got the answer, which is: 35 to 41.
Hope this helps. :-)
