To begin with, we are dealing with place value. For example, 158. 8, is in the ones place, 5 is in the tens place, and 1 is in the hundreds place. To write it out, this simply means 100 + 50 + 8. Referring back to your question, 1 hundred equals 100, and 4 tens equals 40. (Add those) 100 + 40 = 140.
One way to look at 14 tens is 14 x 10. Ten = 10, 14 (10) = 140. So 14 tens equals 1 hundred 4 tens because they both equal 140.