на 20% эффективнее, чем
• 1 g of manganese in the form of ... is 20 percent as effective as (or 20 percent more effective than) the same weight of iron in the form of ...
Источник:
Русско-английский научно-технический словарь
на Gufo.me
• 1 g of manganese in the form of ... is 20 percent as effective as (or 20 percent more effective than) the same weight of iron in the form of ...