Counters - General Questions

If each flip-flop in a 4-bit ripple counter has a 10 ns delay, the worst-case settling time is:

How many flip-flops are needed for a mod-10 (decade) counter?

A Johnson (twisted-ring) counter with n flip-flops has how many states?

A ring counter with 5 flip-flops divides the clock frequency by:

How many unused states does a mod-10 counter built with 4 flip-flops have?