So this needs explaination but to cheer you, this is very easy!
now they are saying y is directly propotional to x.
when y is 20 then x is 4......so in other words y is 4 *5 that is 20.
now let is take the other situation.
when y is 80 then we gotta find x so in other words y is 16 *5 thats 80.
Now that was only for your understanding......direct proportion means that there are two amounts and they are increasing in proportion....that is if suppose amount a has increased...then even amount b will increase...keeping in pace with amount a as before.....
So to write the answer in your notebook we will solve this "direct proportion" problem like this:
$$\frac{x}{y}=\frac{4}{20},\frac{x}{80}$$
(actually there was supposed to be a line instead of a comma......you will put this info in a table)
$$\frac{4}{20}=\frac{x}{80}$$
(this is how we will place both the quantities...now do cross multiplication)
$$20x=80 * 4$$
(to make our work easier we will not multiply now but take 20 to the other side and devide)
$$x=\frac{80*4}{20}$$
(now cut the two zeroes , devide 2 by 8 and multiply the remaining)
$$x = 16$$
so that is our answer 16
to check , devide 16 by 80 and see what you get.........youll get 1/5.........if not....then we'll have an argument....whoever wins will be the ruler.....HAHA!
Hope youve got it..........