Jill's bug collection contains 12 beetles and spiders. Beetles are worth 10 cents and spiders are worth 20 cents. Altogether Jill's collection is worth $1.70. How many beetles and how many spiders does Jill have?
b+s = 12
10b+20s = 170
it's easier if you use
b+s = 12
b+2s = 17
subtract to get s=5
so, b=7