Learn Russian: Геодезист поехал отмерить две пятых гектара. - The surveyor went to measure out two fifths of a hectare.

In Russian, fractions use a cardinal number for the numerator (the top number) and an ordinal number for the denominator (the bottom number). The numerator is feminine because the word for part (часть, a feminine noun) is implied. Since две means two, the denominator goes into the genitive plural (пятых, literally meaning of fifth parts).

If the fraction ends in a plural word (пятых), why is гектара in the genitive singular?

The noun following a fraction always takes the genitive case, but its number (singular or plural) depends on what you are taking a fraction of. Here, the surveyor is measuring two fifths of a single hectare, so it is гектара (genitive singular). If it were two fifths of several hectares, you would use the genitive plural гектаров.

What does the prefix от- add to the verb in отмерить?

The base verb мерить means to measure in a general sense. The prefix от- carries the meaning of separation or detaching a piece from a whole. So, the perfective verb отмерить specifically means to measure out or to section off a designated amount.

Why use the perfective uni-directional verb поехал here?

поехал indicates a single, specific trip that began in the past. Pairing a verb of motion with an infinitive like отмерить is the standard way to express purpose: the surveyor set off on a single trip in order to successfully complete a specific task.