Andy has some counters.
15% of the counters are red.
2/5 of the counters are blue.
the rest of the counters are yellow.
there are 27 yellow counters.
how many blue counters are there?

24 blue counters

you must find what percent is shown by the yellow counters and then calculate the 1% to then multiply by the percent shown by the blue counters to give you the number of counters

15% = 15/100
2/5 = 40/100

15+40 = 55 --> 55/100
100 - 55 = 45

so 27 counters = 45/100

27 divided by 45 = 0.6 (= 1%)
0.6 x 40 = 24