Q.In human faeces, bacteria make up about one-third of the roughly 50 g excreted per day, and each bacterial cell weighs about 1x10^-12 g. Estimate the number of bacterial cells excreted per day.

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The question asks for an estimate of the number of bacterial cells excreted per day, given that bacteria make up about one-third of ~50 g of daily faeces and each cell weighs about 1x10^-12 g. …

The problem states that roughly 50 g of faeces are excreted per day, and that about one-third of this mass is bacterial cells, with each bacterial cell weighing about 1x10^-12 g. The mass of bacteria excreted per day is therefore approximately 50 g / 3 ≈ 16.7 g. To find the number of bacterial cells this represents, divide the total bacterial mass by the mass of a single cell: 16.7 g / (1x10^-12 g per cell) ≈ 1.7x10^13 cells. This works out to roughly seventeen trillion bacterial cells being excreted in human faeces every day, which is a striking illustration of just h …

• Forgetting to take one-third of the 50 g figure before dividing by the per-cell mass, and instead using the full 50 g. …