Here is a website for such things:
https://www.oddsshark.com/tools/odds-calculator
You have to bet $15 to win the $10 at 60%
a bet of $15 to win at 40% should win $22.50 (according to the calculator)
Perhaps ? 60% bet == 15 x 40/60 = $10 win
40% bet == 15 x 60/40 = 22.50 win ??
....or another way to look at it...
Total payout on $15 bet
bet on 60% : 100/60 x $15 = $25 payout ($10 winnings)
bet on 40% : 100/40 x 15 = 37.20 (22.50 winnings)