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