Well the multiplication part is the same .Its in the ordering it has to be(ex: 42 times 35) but it has to be placed vertically .For the second number multiply to the right then the left .Repeat .Then you got your answer using 2 digits similar to regular multiplication partial products .Hope this helped !!
(100+70)/2
hope this helps
85% of 20 is 17
20 x .85 = 17