Two math problems

1) The sum of twice a number and -6 is nine more than the opposite of that number. Find the number.

2) A collection of 27 coins, all nickels, and dimes, is worth $2.10. How many of each coin are there? The dime, in United States usage, is a ten-cent coin. A nickel, in American usage, is a five-cent coin composed of 75% copper and 25% nickel.

Correct result:

a =  5
d =  15
n =  12


2a+(-6) = 9 + (-a)

2•a+(-6) = 9 + (-a)

3a = 15

a = 5

Our simple equation calculator calculates it.

d + n = 27
0.10d + 0.05n = 2.10

d + n = 27
0.10•d + 0.05•n = 2.10

d+n = 27
0.1d+0.05n = 2.1

d = 15
n = 12

Our linear equations calculator calculates it.

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