What is the (a) measure of "c" and (b) what proportion may be written to solve for "c​

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

Campus rentals rents 2 and 3 bedrooms apartments for $700 ans $900 a month respectively. last month they had six vacant apartments and reported $4600 in lost rent. how many of each type of apartment were vacant?

What’s the range cuz i can’t find it?