Find the cost of 100gram at 85p per 1/2 kg?

85p per (1/2) kg.

1 kg = 1000g, (1/2) kg = (1/2) * 1000 = 500 g

It means:

500g cost 85p

1g would cost 85p/500

100g would then cost (85p/500) * 100 = 19p

Cost of 100g is = 19p

Hope this explains it.