Using a 25mL pipette, you prepare a standard solution by pipetting 25.00mL of the standard iron solution[0.25g/L] into a 500mL volumetric flask. Then, using a 10mL pipette you transfer a 12mL aliquot (10mL+2mL) of this solution into a 50mL volumetric flask and dilute up to the mark.

Calculate the % uncertainty.

The correct answer is:   [c]:     "it is the minimum amount of material needed to sustain a fission  reaction."

B)rock pieces are moved from place to place

The rules are different for multiplication and addition, but the least precise value always restricts the answer. first do the multiplication (as per standard order of operations): 24.6681 × 2.38 ≈ 58.7 since 2.38 has the fewest significant figures, it is considered the lease precise value then do the addition: 58.7 + 332.58 ≈ 391.3 since 58.7 has fewest figures right of decimal it is considered least precise. so final answer: 391.3 with four significant figures.