There are two different approaches to solve the problem. One method involves using a method of trial and error; the other method involves using algebra.
1. Trial and Error
Choose a number, for example, 6. Try 6 as the number of presents that Davina has.
6 + (2 x 6) + (2 x 6 + 10)
6 + 12 + (12 + 10) = 40
This leaves 40 which is too much so try a smaller starting number.
This time choose a number smaller than 6, like 4.
4 + (2 x 4 ) + (2 x 4 + 10)
4 + 8 + (8 + 10)
4 + 8 + 18 = 30
2. Algebra Choose a letter of the alphabet to represent the unknown number of presents that Davina has. The letter that has been chosen in this case is n.
Re-organise the ns by counting them all up and grouping them together. Then, whatever calculation is performed on one side of the equation must be done on the other side as well. See the purple calculations.
Always go back and check the calculations with the answer you have found. See the equation below. The white text shows n.
Which method did you use? Hopefully, you'll be able to solve this type of problem next time.