Hi... umm guys m stuck on this question... can sumbdy plz guide me!!

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 replacements in 'a to b minutes'. If this claim is to be correct for 90% of the repairs, find a and b correct to 2 s.f., bassed on a symmetrical interval centred on the mean. (ans: a=80 and b=100)

Can sumbdy plz help asap!!