let 'x' be the number of ten-centavo coins
'y' be the number of five-centavo coins
x + y = 10 ----equation 1
0.10x + 0.05y = 0.85 ----equation 2
from equation 1 you'll have
x + y = 10
x = 10 - y -----equation 3
substitute equation 3 to 2
0.10(10 - y) + 0.05y = 0.85
1 - 0.10y + 0.05y = 0.85
-0.05y = 0.85 - 1
-0.05y = -0.15
y = 3
substitute to equation 3
x = 10 - y
x = 10 - 3
x = 7
