=3*5*7 (xyz)
=15*7 xyz
=105xyz
D. 10 is your answer.
In order to solve a proportion, you cross multiply.
In this case it will be 40=4x
Solve for x is 10. Therefore D is your answer.
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A multiplicative inverse of an integer a is an integer x such that the product ax is congruent to 1 with respect to the modulus m.
1. 
Check:
The multiplicative inverse of 5 in
is 9.
2. 
Check:
The multiplicative inverse of 5 in
is 5.
3. 
Check:
The multiplicative inverse of 5 in
is 8.
Maria = x
Tom = y
Use substitution
x+y=30
2y=x
2y+y=30
3y=30
y=10
Tom is 10 years old.
x+y=30
x+10=30
x=20
Maria is 20 years old.