Chetan wants to make a necklace with 50 beads. he knows that 12 beads take up 5 inches of string, but the store only sells string by the centimeter. how many centimeters should he buy? explain.

1 inch = 2.54 cm

Set up this ratio
12 beads / 5 inches = 50 beads / x inches. worry about the cm later.
12/5 = 50 / x Cross multiply
12x = 250 This is not going to be very nice.
x = 20.833 inches. We better round this later. 

Set up another proportion
1 inch / 2.54 cm = 20.8333 inches / x cm 
1/2.54 = 20.833 / x
x = 2.54 * 20.8333
x = 52.92 cm. Now the question is what to do. He either needs to order 1 more bead or one less.

Place the order and see what the store manager says or obey what you've been told about rounding.