Subnetting a Address

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I need to divide a 193.136.37.0/24 class C network in 4 different subnets, and calculate the address and masks for each subnet.
Subnet A has 10 machines, B has 19, C has 71 and D has 3 machines.

If I understood this well, I've to divide the last 8 bits of the IP address (which identifies the host) so it can also identify the subnet.
2 bits to identify the subnet (there are 4 subnets, 2^2 = 4). 7 bits to identify the hosts, 2^7 = 128 (2^6 is 64 and subnet C need at least 71 addresses so 6 bits won' suffice).
But 2 + 7 = 9, and it must be 8 bits to fit in the subnet mask...
Can someone help me with this one?