To calculate a 15% tip, simply take 10% of the tip, in this case $4.20, then take 50% of that number. 50% of $4.20 is $2.10. Then you add those two numbers together, $4.20+$2.10=$6.30
To do this you have to multiply 60 by 35, and divide that answer by 100, and your answer should that number. In this case, the answer is 21, because 60x35 is 2100, and 2100/100 is 21.