方法一：用方程解． 方法二：用算术方法．

设甲数是x，根据题意得 4.2×

4

7

÷

4

5

=3

4

5

x=4.2×

4

7

=2.4÷

4

5

4

5

x=2.4=2.4×

5

4

x=2.4÷

4

5

=3

x=3

故填3．