Y = axb Where Y = cumulative average time per unit to produce x units a = the time taken for the first unit of output x = the cumulative number of units produced b = the index of learning (log LR/log2) LR = the learning rate as a decimal
When asked to find the learning rate you cannot be expected to use the formula (because with regard to b it would involves using antilogs and they are not in the syllabus).
Finding the learning rate will only ever involve using the doubling rule and I explain this, with examples, in my free lectures on learning curves.