var secondsPerMin = 60;
var amount = prompt('How much money do you have?');

Parsing

parseInt(amount);   // '15.5' returns 15
parseFloat(amount); // '15.5' returns 15.5
parseFloat(amount); // '15.5 dollars' returns 15.5
parseFloat(amount); // 'In dollars: 15.5 ' returns NaN
isNaN(value);

Numbers inside strings

var questionsLeft = ' [' + 3 + ' questions left]'; 

Math Object

Math.round(2.5); // = 3
Math.ceil(2.5); // = 3
Math.floor(2.5); // = 2
Math.random(); // = 0.6824359748673245 (0-0.9999999 float)
Math.random() * 6; // 5.543644742316923 (0-5.9999999 float)
Math.floor( Math.random()*6 ); // 4 (0-5 int)
Math.floor( Math.random()*6 ) + 1 // 
Math.floor( Math.random() * max ) + 1 // 4 (0-6 int) – How usually it's done