Teaser 3131: Heads up savings

From The Sunday Times, 25th September 2022 [link] [link]

Little Spencer saves 5p coins in a jar, and when they reach £10, deposits them in his savings account. He likes playing with the coins. In one game, after first turning them all heads up, he places them in a row on the table. Starting from the left, he then turns over every 2nd coin until he has reached the end of the row. He then again starts from the left, and this time turns over every 3rd coin. He repeats this for every 4th, 5th coin etc, until finally he turned over just one coin, the last in the row.

At the end of the game I could see that if Spencer had exactly 75 per cent more coins he would have an increase of 40 per cent in the number showing heads. However, if he had exactly 50 per cent fewer coins, he would have a decrease of 40 per cent in the number showing heads.

What is the value of his 5p savings?

There are now 750 Teaser puzzles available on the S2T2 site.