Elly, Nathan and Terence each had an equal number of paperweights. Elly had 9 metal weights and some wooden weights. The total mass of Elly's weights was 44.7 g. Terence only had wooden weights. Elly found that the total mass of her weights was 7.2 g more than Terence's weights. Nathan had 4 metal weights and some wooden weights. Find the total mass of Nathan's weights. Ans: ___ g

1. Find the difference in mass between 1 metal weight and 1 wooden weight.

Since Elly and Terence have the same total number of weights, but Elly has $9$ metal weights while Terence has only wooden weights, the $7.2\text{ g}$ difference is due to $9$ metal weights replacing $9$ wooden weights.
Difference between $9$ metal and $9$ wooden weights $= 7.2\text{ g}$
Difference between $1$ metal and $1$ wooden weight $= 7.2 \div 9 = 0.8\text{ g}$

2. Find the total mass of Terence's weights.

Terence's total mass $= 44.7 - 7.2 = 37.5\text{ g}$

3. Calculate the total mass of Nathan's weights.

Nathan has the same total number of weights as Terence, but with $4$ metal weights instead of wooden ones.
Nathan's total mass $= 37.5 + (4 \times 0.8) = 37.5 + 3.2 = 40.7\text{ g}$