The answer is 150 people like the current benefits plan.
If you convert 5 out of 8 and make the denominator 16 also, you'll have 10 out of 16. Adding the new converted fraction to the 3 out of 16 people who liked the benefits plan, you'll have 13 out of 16. Seeing that 3 out of 16 did not vote and they're 150 of them, making 3/16=150. Therefore the 3 out of 16 people who did like the plan will have to be 150 employees also.
