The value of 15 tens would be 15 tens added together. Instead of adding 15 to itself 15 times, the process can be shortened by using multiplication. Multiplication basically takes a number (10) and adds it together a specified amount of times (15) with only one step.
15 × 10 = 150
By the way, when multiplying something by 10, you can simply add a zero onto the end of the other number.
