There are all sorts of techniques which children can learn to help them with mental arithmetic. One of them is to able to multiply a 2-digit number by 5. This can be done in the traditional method similar to the written method but holding parts of the answer in your brain whilst working out the next part. However, it is often much easier to multiply the number by ten and halve the answer. So: 27 x 5 is done by 27 x 10 = 270. half of 270 is 135. 27 x 5 = 135. Obviously confidence with halving numbers is important here. If the number to be multiplied is even then the process can be swapped round. So: 28 x 5 can be done by halving 28 which is 14. Then 14 x 10 = 140. 28 x 5 = 140.