Making 3-digit numbers

In your book, Maths Mela (Chapter 4), you make 3-digit numbers using only the digits 3 and 7. You may use a digit more than once. The book starts you off with 333.

Let us find them all in order. Numbers starting with 3: 333, 337, 373, 377. Numbers starting with 7: 733, 737, 773, 777. That is 8 numbers. The smallest is 333 and the largest is 777. To order them, look at the hundreds first, then the tens, then the ones. 377 is smaller than 733 because 3 hundreds is less than 7 hundreds.

Next the book gives the digits 3, 5, 0 and 8 and asks for 3-digit numbers that are less than 550. A 3-digit number cannot start with 0, so it starts with 3, 5 or 8. Any number starting with 8 is 800 or more, which is too big. Numbers starting with 3, like 305, 358 and 380, are all less than 550. Numbers starting with 5 need care: 508 and 538 are less than 550, but 553 and 558 are more than 550.

The book also asks you to continue counting. 787, 788, 789, 790: each is one more. 338, 348, 358, 368: here each number is ten more, so the tens digit goes up. 679, 689, 699, 709: ten more each time, and after 699 we cross into the 700s.