Question

The time taken by a garage to replace worn-out brake pads follows a normal distribution with mean 90 minutes and standard deviation 5.8 minutes. The garage claims to complete the replacement in 'atobminutes'. If this claim is to be correct for 90% of the repairs, findaandbcorrect to 2 significant figures, based on a symmetrical intervals centred on the mean.

Attempt:

I only know that and . I can't understand what the question exactly means. How am I suppose to find the values ofaandb?