Are the following ratios proportional? Explain why or why not. 3 to 8; 7:20

Nope! If they were proportional, one would be able to be reduced to the other.

It's like the (however clichéd) example of pie slices! If you cut two pies into 8 and 20 pieces, three pieces of the pie sliced into eight pieces will not be the same as seven pieces of the pie sliced into twenty!