Using the table of energy stored in 100 g of foods (apple 200 J, banana 340 J, chips 1 kJ, cooked beef 1 kJ, chocolate 1.5 kJ), what total energy is consumed by eating 50 g of chocolate and 200 g of banana?

What you do with a “per 100 g” energy table

The table values are for 100 g servings, so you scale each food’s energy in proportion to the mass you actually eat, then add the energies.

Energy from 50 g of chocolate

Chocolate has $1.5\,\text{kJ}$ per $100\,\text{g}$.

50 g is half of 100 g, so: $$E_{\text{choc}} = \frac{50}{100}\times 1.5\,\text{kJ} = 0.5\times 1.5\,\text{kJ} = 0.75\,\text{kJ}$$ (That is $0.75\,\text{kJ} = 750\,\text{J}$.)

Energy from 200 g of banana

Banana has $340\,\text{J}$ per $100\,\text{g}$.

200 g is double 100 g, so: $$E_{\text{banana}} = \frac{200}{100}\times 340\,\text{J} = 2\times 340\,\text{J} = 680\,\text{J}$$ (That is $680\,\text{J} = 0.68\,\text{kJ}$.)

Add them in the same unit

Convert to the same unit and add: $$E_{\text{total}} = 0.75\,\text{kJ} + 0.68\,\text{kJ} = 1.43\,\text{kJ} = 1430\,\text{J}$$