Are two equal payments of size x equivalent to a single payment of 2x made midway between the two scheduled payments? If not, is the equivalent payment larger or smaller than 2x? Explain.

No-two payments each of size x have a larger economic value than a single payment of 2x made midway between the two payments. This can be demonstrated by a general algebraic proof or by working through a particular numerical example. However, these approaches may not provide students with an intuitive understanding of why the first alternative is more valuable than the second.
For an "arm-waving" explanation, divide the 2x payment into two halves. Then separately compare the economic value of one half with the first x payment, and the economic value of the other half with the second x payment.
Clearly, x paid sooner has a greater economic value than x paid later. The first x payment is worth more than the first half of the 2x payment. The second x payment is worth less than the other half of the 2x payment. The two differences have the same magnitude. However, they are not a perfect offset or "wash" in terms of economic value because the earlier advantage of the first x payment can be invested during the second half of the interval. The growth opportunity of this initial advantage up to the time of the final x payment is the source of the slightly higher economic value of the separate x payments.