Peter has 35 stamps. Some are valued at 2 cents, others at 5-cents, and the remainder at 10 cents. he has three times as many 5-cent stamps as he has 2-cent stamps and the total value of all stamps is $1.89. How many of each stamp does he have?
He has three times as many 5-cent and 2-cent stamps. If he has 2-cent stamps, how many 5-cent stamps is that? (You need to multiply.) So how much will they be worth altogether? (You need to multiply again.)
Now he has 35 stamps altogether. To find out how many 10-cent stamps, add together the number (not their value) of 2-cent and 5-cent stamps (that's plus whatever number of 5-cent stamps he has), and take the total away from 35. This will give you something like 35 - (something) . Now multiply this number by 10 to find out how much they are worth.
Then: add together the value of all the stamps, and put the total equal to 189. So you'll get an equation like:
+ (something) + (something-else) = 189.
Then solve this equation for 's, and you're there.
Can you do that?