A network on the internet has a subnet mask of 255.255.240.0. what is the maximum number of hosts it can handle

It is a class B network, so for a class B network, the upper 16 bits form the network address and the lower 16 bits are subnet and host fields. Of the lower 16 bits, most significant 4 bits are 1111. This leaves 12 bits for the host number. So, 4096 (2^12) host address exists. First and last address are special so the maximum number of address is 4096-2=4094.