The product of 15 and a number (when the number is n) is 15n. Subtract 8 from this and we have 15n-8
addition a number
Is a number being multiplied by another number.
In a subtraction sum, for example, 8 - 3 = 5, the first number is called the minuend. The second number is called the subtrahend, while the third number is the difference. Therefore, the number from which you are subtracting is called the minuend.
A number being added to another number is called an addend. The total of the numbers added together is called the sum.
Simply reverse the question and you find that 4 + 6, which is 10 equals half of the unknown number. 10 x 2 = 20, which must therefore be the unknown number.