Compound operators combine arithmetic with assignment. x += 5; means x = x + 5;. This shorthand is faster to write when modifying a variable. All arithmetic operators have compound forms: +=, -=, *=, /=, %=.
count *= 2; doubles count. total /= 3; divides total by three. Compound operators follow the same type rules. int x = 10; x /= 4; uses integer division, storing 2 not 2.5.
Convert to double first if you need decimals.