Robert bought $1.60 worth of stamps with 20 coins all in nickels and dimes. how many nickels and dimes did he use?

Nickels are 5 cents, and dimes are 10 cents.
(By luck..) eventually you can have 12 dimes, meaning you have 120 cents, or $1.20 ... 20-12=8.. meaning you used 12 dimes and are missing 8 more coins. which means you can have 8 nickels which is 40 cents. 120+40=160 cents or $1.60
Hope this helped.