Verified Answer :

  1. a^2 + b^2 = c^2...where a and b are the legs and c is the hypotenuse
        7^2 + 10^2 = c^2
        49 + 100 =c^2
        149 = c^2....take square root of both sides, eliminating the ^2
         sq rt 149 = c
         12.21 = c.....so the approximate length of 3rd side is 12 m

    6. x = number of books bought

    7. m < 2 + m < 8 = 180

    8. 1000 = 400 + 24x
         1000 - 400 = 24x
         600 = 24x
         600/24 = x
         25 = x <=== 25 months he will have spent $ 1000