What is the difference between average speed and instantaneous speed?

Can average speed and instantaneous speed be equal? Explain.

Can be a body be accelerated, if it is moving with constant speed?

If a body covers a distance $x$ with speed $v_1$ and then it covers another distance $x$ with speed $v_2$, find the average speed for the whole journey.

If a body covers three equal distances with speeds $v_1$,$v_2$ and $v_3$ then what is the average speed of the body?

If a body travels for time $\text{'}t\text{'}$ with speed $v_1$ and then it further travels for time $\text{'}t\text{'}$ with speed $v_2$, find the average speed.

Convert $5 \mathrm{m}/\min$ in (a) $\mathrm{m} {\mathrm{s}}^{-1}$ and (b) $\mathrm{km} {\mathrm{h}}^{-1}$