A 4 bit ripple counter and a 4 bit synchronous counter are made using flip-flops having a propagation delay of 10 ns each. If the worst case delay in the ripple counter and the synchronous counter be R and S respectively, then

The digital block in the figure is realized using two positive edge triggered D flip-flops. Assume that for t < t₀, Q₁ = Q₂ =0. The circuit in the digital block is given by

A sequential circuit using D flip-flop and logic gates is shown in the figure, where X and Y are the inputs and Z is the output. The circuit is

In the figure, the J and K inputs of all the four Flip-Flops are made high. The frequency of the signal at output Y is