19
It is the same as: -9 +10 = 1
19/20 and a lot more
20
They are the same.
9 - (-10) = 9 + 10 = 19
9 plus 10 = 19
19
9/10 or 90/100 if you take it literally.
It is probably impossible to enumerate all the methods. Apart from long multiplication you can have Two-way partitioning: (10 + 9) * (20 + 1) = 200 + 10 + 180 + 9 = 399 This is usually done in a 2x2 grid. Partitioning 21 19*(20 + 1) = 19*2*10 + 19*1 = 38*10 + 19 = 380 + 19 = 399 Partitioning 19 (20 - 1)*21 = 2*10*21 - 1*21 = 42*10 - 21 = 420 - 21 = 399 Or. if you are savvy enough, Difference of two squares (20 - 1)*(20 + 1) = 202 - 11 = 400 - 1 = 399
19
yes 10 + 9 = 19