To express a number as a percentage, you need to write it as a ratio with denominator 100.
We know that ratios have different forms they can be written into, for example, all the following are equivalent:
So, since 43 students out of 860 are sick, the ratio of sick students is
and we want to express this as a ratio with denominator 100, so we have
So, if we solve for x, we have
So, 5% of students is sick.