This is the concept of simultaneous equation, suppose the number of nickels is x and that of dimes is y;
thus the total number of coins will be:
x+y=30......i
the total amount will be:
0.05x+0.1y=2.50....ii
from i
x=30-y.....iii
substituting iii in ii we get:
0.05(30-y)+0.1y=2.50
1.5-0.05y+0.1y=2.50
1.5+0.05y=2.50
0.05y=2.50-1.5
0.05y=1
y=1/0.05
y=20
x=30-20=10
therefore we conclude that there are 10 nickels and 20 dimes
