The object of this chapter is to encourage you to think whether or not your answer to a problem looks reasonable or ridiculous. In general, a reasonable answer is likely to be a right answer. An answer that looks ridiculous might also be right, but you should then be alert to check your calculation very carefully. Of course, there will be times when you do not know what to make of an answer – is it reasonable or is it not? The better your background of knowledge and experience, the less often will this uncertainty happen.

A ridiculous answer that is wrong

Here is the problem: calculate what dry weight of bacteria will be present in 10 litres of medium in a fermenter after 10 h when at time zero there are 10 organisms ml−1 and there is a lag of 1 h before exponential growth (doubling time 20 min) begins. One organism has a dry weight of 1 × 10−12 g.