Problem 41. To cut without a ruler

How can one cut off a piece of ribbon 1/2 of a yard long from the piece of ribbon 2/3 of a yard long without a ruler?

Problem 40. The last digit

What is the last digit of 71000 ?

Problem 39. The new average

The average of seven numbers is 6. If 1 is added to the first number, 2 is added to the second number, 3 is added to the third number and so on up to the seventh number, what is the new average?

Problem 38. Straight lines

There are 10 points on a plane and no three of them lay on the same straight line. Through each 2 points a straight line passes. How many straight lines are there?