The format is as follows: 1 bit sign | 6 bits exponent | 10 bits for mantissa
Now, lets focus on exponent. The range of exponent (without any bias) will be -32 to 31. But, these extreme values are reserved. So, highest value of exponent will be 30.
Now, lets consider normalized manitissa. The max value will be 1.1111111111.
1) The fraction part of 1.1111111111 evaluates as ($2^{-1}+2^{-2}+2^{-3}...+2^{-10} = 2^{-10} *(2^9+2^8...+2^0) = 2^{-10} *(2^{10}-1) =2^{-10} *(2^{10}) =1$)
2) We also need to add the 1 to the left of the fraction 1.1111111111 which is 1.
So, total = 1) + 2) =2
So, max number that can be represented is = $2^{30}$ * 1.1111111111 = $2^{30}$ * 2 = $2^{31}$