Hey there! :D
There is an additional charge for each topping.
$16= 1-topping pizza with basic charge.
$20= 3-topping pizza with basic charge.
The difference is:
20-16= 4 (the difference between the one topping and three topping pizza)
4/2=2 <== the price for each topping (divide the difference of toppings)
So, 16-2= 14 <== basic charge.
Price for 5-topping pizza:
14 (basic charge)+ 2(5)
14+10
24
The 5-topping pizza is $24 dollars.
I hope this helps!
~kaikers