Estimate

In the context of the exam questions this means that: you need to assign an approximate value. This is not the same as a guess, your scientific knowledge may allow you to use a sensible method for making the estimate. 

For example, you may need to apply simple arithmetic to some related data or follow the trend of a line on a graph. 

There is additional information on this within the Maths skills pt1 tutorial, which is within the 4.1.1.5 Microscopy lesson.