We subtract because we are taking something away!
Hope this helps!
~Learning with Natalia~
To solve such questions you must be aware about "subtraction." :D
In simple words, subtraction is the deduction of a smaller value from a larger value whose resulting value is always positive. Not always positive, it can also be negative depending on the values given in the problem.
1) There are 35 students in a bus. 12 of them are girls. How many boys are there?
Total number of students = Number of girls + Number of boys
35 = 12 + Number of boys
35 - 12 = Number of boys
23 = Number of boys
Hence, out of 35 students 23 of them are boys.
2) Tom sees 16 birds in a farm. He scares 12 of them away. How many left?
Total number of birds in a farm = Birds scared by Tom + Birds left
16 = 12 - Birds left
16 - 12 = Birds left
4 = Birds left
Hence the number of birds left is 4.
3) There were 33 flowers on the ground. Sita picks 18 of them. How many flowers are still on the ground?
Total number of flowers = Flowers picked by Sits + Flowers left on the ground
33 = 18 + Flowers left on the ground
33 - 18 = Flowers left on the ground
15 = Flowers left on the ground
Hence, 15 flowers are still on the ground.
4) John had 50 pieces of building blocks. He used 15 of them to make a toy car. How many left?
Total pieces of building blocks = Pieces used by John to make a toy car + Pieces left
50 = 15 - Pieces left
50 - 15 = Pieces left
35 = Pieces left
Hence, 35 pieces are left.