I really need help on this I’m stuck

2-digit numbers less than 91 which are 1 less than a multiple of 10

Arley is building a model of a city map. in one part of the city, three roads form a right triangle, which harley draws as triangle abc, with the following measures: m∠b=90° and m∠a=30°. in his scale model, the hypotenuse of triangle abc, ac¯¯¯¯¯¯¯¯, has a length of 817−−√ cm. what is the value of a (the length of bc¯¯¯¯¯¯¯¯)?

Ling listed all of the factors of a mystery number: 1,2,4,8,16,32 what is the mystery number?