The problem tells you that Mrs. Margul wants to leave a #15%# tip to her hairdresser, so the first thing to do here is to use this information to calculate the value of the tip.
Now, a #color(blue)(15)color(darkgreen)(%)# tip basically means that Mrs. Margul will pay #color(blue)($15)# for every #color(darkgreen)($100)# in her bill. In essence, you can treat a percentage as a conversion factor to take you from the value of the bill to the value of the tip or vice versa.
In this case, the bill is #$125#, so Mrs. Margul will have to add a tip of
#$125 color(red)(cancel(color(black)("bill"))) * color(blue)("$15 tip")/(color(darkgreen)("$100" color(red)(cancel(color(black)("bill"))))) = "$18.75 tip"#
This means that the total bill, i.e. bill + tip, will amount to
#"total bill" = $125 + $18.75 = $143.75#